2nd video card / new video card for Crysis

[TheSalche]

Current system specs: AMD dual-core 4600+, 2 gigs of ram, X1950xt video card 256 mb, monitor is 22" widescreen with 1680x1050 res

From what I've heard, Crysis won't run well on my video card, and from when I played the demo I agree with this. At the very least, the game won't look as it is meant to. So I'm considering getting a second video card or a replacement card. Here's my questions:

1. How will an additional, cheaper video card similar to the one I own help my system?
2. If I get a more powerful video card (i.e. 8800) should I keep my old one?
3. How well do ATI and nVidia cards mix?

Oh, and my slots are PCI express 16x