640 x 480???????? That's going to look HORRIBLE! And even a HDTV with a VGA connection should be able to do better than that! LOL
LOL You could put just about any video card in to play @ 640 x 480.
Any 20" PC Monitor these days (should you buy one) will be 1680x1050 (minimum) or 1920x1080 resolution.
I'd suggest the GTX 460 if you can afford one. That'll do better up to 1920x1080. If the GTS 450 is your budget cap, I'd get a 1680x1050 monitor as 1920x1080 will be pushing it with the latest game releases.