How important is my Video Card?

[cassette]

Ok, so I am giving in and upgrading my poker machine into something that can play games. Half Life, Civ 4, ect.

I don't know anything about video cards.

What's the difference between PCI, PCI Express and AGP cards?

Would either of these two fit my needs? What would be the noticable differences between them?

Leadtek Geforce 6600GT 128MB (Nvidia,16x PCI-E w/ TV-Out, DVI, SLI) $202
ATI All in Wonder Radeon 9800 Pro 128MB (ATL, AGP 8x TV Tuner, Video Capture, Remote, TV-Out) $269

Would I be able to hook up 2 monitors to either of these?

Thanks for any help.