Definitly your gfx card, I would recommend the 8800GT or the new 512MB version of the 8800gts.
If you are ready to spend quite a lot of money and wait until next month, I suggest you get the GeForce 9800 that will be released next month.
I doubt that you have AGP gfx ports but IF you do, then the ATI Radeon x1950pro would be a good choise.
A tip, check on Toms hardware(
www.tomshardware.com) there u can see a chart with gfx cards performances and much more.
Use the menu to the left and click "Graphics & displays" in the sub-menu select "VGA charts", you can select two gfx cards to compare(marked with blue on the chart) and nvidia cards green and ati cards red. And also select how to compare them, several games with different settings and resolutions in each game, overall all games fps and also 3DMark06 with different settings and resolutions