Video Card

Depends a lot on some other specs and budget, etc. Especially if it's an older or lower-end system.

I only saw the current card in PCI-E. but to be sure:
Does the system run PCI-Express video cards? AGP?

For other specs, how much RAM and what CPU do you have? If those are real weak, it could be better to upgrade those.

Offhand, I prefer Nvidia's 8000 and 9000 series.

If you just need some thing that can run most games and not be perfect i say go for a ATI there half the price of a Nvidea one and still run good.

The best price : performance card out there at the moment is the ATI radeon 4850. This card will eat sins alive, and also burn through any other high end game out there, at whatever resolution your monitor will support (with the possible exception of Crysis :p)

ATI have really punished nvidia this generation, in terms of providing high end cards at affordable prices. This is good, since nvidia have been pretty much on top for the past 2 years!

I think the reason ATI has the edge on NVIDIA currently is the amount of resources the latter has channeled into the development of SLI and PhysX technology, as opposed to strict GPU development. Sure, ATI has some good cards, but it's hard to beat two Geforce series cards in SLI with a PhysX processor.
Obviously, if you're looking for value, that's not the way to go (2x the power, 2x the price, plus an SLI bridge).

would just like to add to debate yes ati are offering great performance at affordable prices however you dont need to resort to sli for staggering performance have just got nvidia 280 asus graphics card and it romps every game i own,unfortunatley the price is high but they are falling and quickly

SLI and Crossfire both have subtle performance issues, mainly stemming from irregular frame output (often called 'microstuttering'). The bottom line is that 40fps (say) on a dual-GPU setup does not appear as smooth as 40fps on a single GPU setup. This is quite aside from other compatability issues.

I've had multi-GPU setups in the past, but no more for me. I also just went for a GTX280 now that the price has dropped significantly, but I certainly wouldn't recommend it to a value-concious buyer. ATI win out in this arena.

What's the best SLi mode to run Sins? I've been searching the forums, but have come up empty-handed..

If you have a few grand to spend, You could get the Dell XPS 720H2C,

Quad Processors, Liquid Cooled Video card/Processor, Windows Vista Ultimate, 4GB RAM, with that you can run Sins on Maximum in a Huge Multi Galaxy Map.

If you're having virtual memory problems, that means that the computer is running out of RAM and using the HDD (which is MUCH slower) as a replacement. To remedy this, but 1 or 2 GB more RAM.

If you have a PCIE slot, the best bang for the buck you can get with a card these days is either an 8800 GT or an 8800 GTS g92.

What he says.

Virtual Memory is a windows setting which I usually set to be 1.0 to 1.5 times that of RAM. Lower number = upper

It was a nice discourse of video cards though dudes...  

i have a XFX Geforce 8800GT 512mb (Alpha Dog Edition)
Its a really powerful card only cost me $200 including tax. I can play this game on high with no lag at all. This is off topic but i can also play Crysis on high settings with an adverage of 50 frames per second.      

Did you know, the human eye can process approximately 18-28 FPS at most.

I can get 30+ fps on sins with all settings maxed out on the biggest maps you can find.Running 1680*1050

Hardware:
Intel E8400 Core 2 Duo @3.02 Ghz
3GB Ram
Windows Vista Premium x64
XFX GeDorce 8800 Gt 512mb (Alpha Dog Edition)
Creative soundblaster Xfi-Extreme Gamer

I would Reccomend at least a stock 8800 GT for anyone who is going to upgrade, don't bother with the 9000 series, they only offer better performance under DX10, the 260-280 series can smoke almost anything out there, But they are pricey

sounds like your issue is that your hard drive is running out if space. your pagefile.