Sorry there was a typo, repeat onboard memory on the video card won't be a limiting factor ( Don't worry about the onboard ram) That only effects high performance chipsets and cores which benefit from higher memory speeds and bandwidths of GDDR3 (Graphics RAM extremely fast) such video cards are the 6800gt, 6800 ultra, and 7800gt and gtx. As the Radeon X800, PE 850 XT PE. Which would play games like Doom 3, F.E.A.R and Half Life 2
Worry about the chipset and the processor and the support for future games which the Nvidia one has not the ATi 9 series (except for high end cards like 9800's which doesn't seem like an option for you though
Make your choice, young padawan.......
Oh yeah good luck and good gaming!!!!