1) I can't think of any. What you should be more concerned about is to make sure that whatever graphics card you get is compatible with your Motherboard. This goes for everything, by the way. Don't throw hundreds of dollars down the drain just because you 'expected' 4 1GB RAM-sticks to work, just because 512MB-sticks of the exact same kind does, and the board works for other 1GB sticks. Yes, I've been there.

2) I want to say "Slim to none", but it depends a lot on wheter or not you have any idea what you're doing. I've burnt a number of graphics cards (and various other pieces of hardware) and I've built a fair number of computers from scratch. I've never, ever, managed to ruin my computer when switching something out. Just be careful, don't touch anything that doesn't need touching, follow the directions, and you should be fine.

3) Everything. Almost anything could potentially be a bottleneck, depending on the game and how big the 'gap' is between the pieces of hardware. Just because you've got 3x RADEON 9800 running in SLi doesn't mean you can run Crysis, if you've only got 256 MB of RAM.

Edit: As Philly0381 said while I was writing this, don't be in a hurry. I'm sure there are people that can put together or work on a computer at very high speeds, putting in things and tearing others out like it's childsplay, but I can take several hours or a whole day to put together a computer, even more if it's entirely from scratch, with installing everything after putting the computer together. So just take your time, don't be a hurry and don't let some idiot stress you up. Even if it means letting everything just lie for the night and pick it up the next morning, it's far better than realizing that something is broken and you need to get a new piece of it, at 11pm. Yeah, I've done that too.

1) Think everything should work with XP, but you would get no DirectX 10 support. Not many games use DirectX10, only Halo 2 and Shadowrun need it, and that's just Microsoft being prats, not that anyone cares about those games on PC.

2) Chances of damaging your PC are low. Turn off the power at the mains, but leave your PC plugged in so that the case is Earthed. You can then touch the case to Earth yourself, or use an anti-static wrist band.

3) Graphics card (best you can afford), then memory capacity (get 4GB), then cpu (get a dual core or better, 2.4Ghz or better).  Also hard drive speed and memory speed may play a minor role, but not enough to be worth spending extra money on them unless you've got the best of everything else first.

When I say only Halo 2 and Shadowrun need DX 10, I mean that they're the only games that don't support DX 9 at all and so won't officially run on XP (although I hear there is a crack to make them do so now). Sorry for any confusion there.

My Windows XP box is using closer to 3 1/2 Gig. of RAM.  I like it a lot better just upgrading to 4 and letting the 1/2 Gig. go to waste.

I will just say that when changing out components in your computer, never ever wear a fleece shirt.  I recommend you wear a normal cotton t-shirt.  The reason for this is that certain kinds of shirts, especially fleece ones, have a tendency to build up static charges which can easily destroy a computer component the instant you touch it.  I've destroyed several computer components simply by not paying attention to what I was wearing.

Having built several computers from the ground up, I would suggest putting it together somewhere that you are not walking across carpeting and that you wear shoes while working.  Stocking feet and synthetic fibers tend to accumulate static electricity.  Personally, I always unplug the computer from any power supply just in case... 

You might check your motherboard manual to see what types of graphic cards it will support before you purchase a new card.  If your board has been around since 2005 as you say your OS is, you might want to look into getting a new one as well.  Also check your power supply as you may need different connections for a PCI/PCIe card that are not available on the older power supply models.

I am currently running Windows XP (32 bit) SP3 with an AMD Athlon 64 dual core processor with a GeForce 8800 GT video card using only 2 gigs of ram and am able to play most any game without problems.  I can play Bioshock without DX 10, but tried Lost Planet and no go. 

Best of luck with your project, just be sure to do a little research before purchasing anything.

...

Please tell me you're joking.

For what it's worth, yes, PCI-E 2.0 cards are backwards compatible to work in PCI-E (1.0) slots.  But, obviously, a PCI-only system or an AGP system is an entirely different ballgame.

Am I correct in believing that an SLI-compatible motherboard is not a requirement if I only care to run one video card?   It said on the box that it needs an SLI-compatible motherboard, but I think that's only if you want to run SLI.

It's a GT, 1 Gig..  It looks like a kick-butt card, too, and for only $160; but not so great of a deal when you pour a $100 power supply on top of that.  Sad thing is, this was originally all supposed to be to play Supreme Commander, which costs a whopping $10. 

9800GT is not quite the top of the line from nVidia at the moment, and I doubt you need something quite that powerful to play Supreme Commander; in fact, I strongly suspect an 8500 or 8600 series would play it just fine.

Kaltes said:

But why use a nvidia card?

Some people are comfortable with nVidia, some with ATI. Personally, I've used a ton of nVidia cards through the years, and I've been satisfied with the performance of every one.

I know, but it was on the shelf for $160, and it came with a $20 rebate and a complimentary copy of Front Lines.  I figured I could keep using it on my next computer.  Only problem is, I may be already looking at getting a "next computer" now, because of the power supply.  That or, get a lower-end video card that won't still be useful after the ugprade (and not as good a deal).   I've already invested $80 to upgrade to 4gig RAM.   I was kinda hoping to wait till DDR3 motherboards started coming out in force before upgrading the computer.

I'm not religious toward either ATI or Nvidia.  I lean toward Nvidia, though, because of the corporate culture.  ATI's going through a lot of stuff right now with AMD.

Yeah, it's a prebuilt.  I've always found those cheaper, because they get the OEM versions of Windows.  I'm also worried about cooling if I upgrade the PSU.  There's a PSU fan, a case fan, a CPU fan, and the video card fan.   I'm sure it will cool just fine all winter long....

I'm running onboard video Nvidia 6150SE. Ancient, but that and 4gig DDR2 run Galciv2 just fine.