It really matters if you have agp or pcie as to what we recommend in the graphicvs card department. Also pcie has faster cards for way cheaper because agp is an old legacy tech.
Visually you can inspect (not recommended):
PCI-E slot is located apprx. 1 1/2" from the M/B edge and slot is apprx. 4" long including the locking tabs
AGP slot is located apprx. 2 3/8" from the M/B edge and slot is apprx. 3" long
PCI slot is also 1 1/2" in from the M/B edge but the length is 3 1/4"
However, I would recommend you download CPUZ and look at the mainboard tab, then to the graphic interface area.
Another thing you may need to take into consideration besides M/B slots, is will your present Power Supply handle what you want to upgrade to and does it have a spare 12v rail?
Also where do you live? In the US (and the rest of the world), graphics cards have never had a better bang for buck as they do now. I would recommend different cards based on your above answers and your budget, but would be hard pressed to suggest less than the 4830.