All of the answers above are correct, but ill dive deeper in the differences in both physical and performance. Ill try to not make it confusing.
Let me start off with the 9800 series of graphics cards by nvidia. These are the entry level main stream cards that perform well enough for the casual and budget gamer. But some dont know this, the 9800 series cards such as the 9800 gtx contain the very same gpu die as the older 8800 series, 8800gtx as an example. The question that may rise is "whats the difference in performance? power consumption?" Aside from memory differences, the mere difference is a mere die shrink, correct me if im wrong, from something larger down to a 65nm manufacturing process. This has its many benifits such as adding more transisters for increased performance, lower power consumption which leads to less heat which in turn allows substantialy higher clock speeds. So in the end, a 9800 gtx is a highly overclocked 8800gtx in terms of only the gpu itself, not the memory. Now the second part of a graphics card, the memory. Memory, also known as vram and frame buffer, in high amounts, such as 512mb gddr3 in the 9800gtx, is what alows you to enable all that eye candy, Anti ailising and Anthroscopic Filtering (pardon the horrible spelling) while still getting the throughput needed to maintain a constant frame rate. At you resolution of 1440-900, to be honest, is more CPU bound than graphics card but the 8600 is severly lacking the oomph needed for today's and future games. In the end, more memory + faster memory speeds = better scaling with AA and AF enabled.
And to Tiduz, to correct you, the g92 gpu die in th 9800gtx+ and the gt200 gpu are vastly different. The gt200 architecture is superior in effeciency, raw speed and power consumption.
All in all id rather give someone in the market for a new card some advice to help em make their choice rather than make a flat out suggestion. I personally have a ATI 4870 1gb w/ a 24" moniter 1900-1200 resolution and the only game i get sub 30 fps is the fabled crysis at max settings. I can play sins with 24xAA and 16x AF w/o a hiccup.
My one suggestion i will say is unless you desperatly need a new(er) card, wait for the release of Windows 7 which introduces DirectX 11, which is substantially better than DX10 an DX10.1. In the mean time, take a look into other upgrades such as adding more memory, faster CPU, and most importantly, a NON bargin powersupply. My old room mate bought my old 4870 512mb and q6600 and ended up frying em cuz he went cheap on a cheap 500w psu.
sorry about the length, this is the kinda stuff that pays my tuition bills so i know quite a bit. any questions, feel free to ask.
