Honestly, if you already have a video card then you don't need a new one. That being said, I have heard frequent claims that ATI cards have better image quality than nVidia cards, the few times that I have compared I have seen little difference, but the claims persist.
So since speed is no concern I would say that you should check to see if you have PCI, PCI-e or AGP and buy the cheapest card of the current generation (i.e. GeForce 7000 series, ATI X1000 series), that you can find in the newest slot that your computer has (PCI-e>AGP>PCI). And since ATI supposedly has better image quality if the nVidia card and ATI card come out to the same price I'd say get ATI.