I have both AGP and PCI-Express slots on my AS Rock 939Dual-Sata2 motherboard. I am currently using an ATI X800 pro graphics card on AGP. I am buying a PCI-Express Nvidia 7950 GX2. Can I use both cards together for better performance? I heard from someone I can use 2 graphics cards on my motherboard at once.

you CAN use both graphic cards at once, but that's called SLI or Crossfire. SLI for nVidia and Crossfire for ATI. it can only be done with two PCIE 16x slots. i sincerely doubt AGP and PCIE can work together.

comparing x800 and 7950... i think 7950 should be more than enough, as it has 2 GPUs in it as you probably already know.

Nvidia is SLi, so are all 7950 GX2s SLi compatible? If not how do I tell if they are?

SLI requires a motherboard that supports SLI.

This board will have 2 (or more) PCI_E X16 slots side by side.

Then you need 2 (or more) SLI PCI_E cards which need to be identical.

the same cards. like 2 7950 gx2s? I was thinking when I go to upgrade my graphics card again I could still use the old PCI-E card in the other slot. not possible if it is a different card?

you need the same card. meaning another 7950GX2 in your case. however, keep in mind though that the nVidia 7950 runs on 2 GPUs, Graphic Processor Units if i'm not mistaken, so running 2 7950 is close to running Quad-SLI.

1x7950 = close to normal SLI, or 2 graphic cards running at once
2x7950 = close to Quad SLI, or 4 graphic cards running at once.

cause the 7950 has 2 GPUs. which means... u need a powerful motherboard, a powerful PSU and alot of RAM.

I have two 2.2ghz athlon 64 x2 cpus 4200

windows xp 64bit

2 gig PC3200

480Watt PSU

and with the 7950 GX2 in the post will my system be fast enough to run the latest games at max details with a 1280 X 1024 resolution? full Anti-aliasing and everything?