http://www.hardocp.com/article.html?art=NTM3Check their performance charts - niether card at the "top end" does badly, but for the same price level (actually, ATI a touch cheaper), with all eyecandy turned on, the ATI still kicks Nvidia's ass.
http://www.newegg.com/app/viewproduct.asp?description=14-102-304 if you want a 9800Pro - 305.00 - only 5.00 past your limit.
No matter what card you get, you'll have to uninstall the old drivers. (get latest reference drivers from nVidia & ATI websites)
Your installation procedure for WinXP will go something like this:
Right click desktop, select "properties", select "settings", select "advanced", select "video card", select "driver", select "uninstall" (am at work at the moment, so series of commands might not be exact, but its close)
Shut down system, check manual for how to disable onboard video if you have onboard video (either jumpre or BIOS, if jumper, short it now, if bios, wait till first restart), or if you have a vid card remove it from the AGP slot, then install your new card into the slot. Restart system, (at this point, if you need to disable onboard video in the BIOS, immediately go to the BIOS to turn it off) Windows will detect new hardware on startup, cancel any automatic driver installation procedure. Then install new reference drivers by hand.
Not sure about nVidia, but the ATI drivers are a .exe file that does everything for you, just start the installation and go.
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