I'm running an Athlon 64 3200+, 1gb memory, and a 256mb radeon 9600 pro overclocked to 450mhz for both core and memory. I've got a regular 7200rpm hard drive. Will I be able to run the game reasonably at the lowest settings?

You don't even need that. Like I said, I'm on a radeon 9600 pro (256 mb), a gig of pc3200, and an athlon 64 3200+. And lolgubbe: Shut your face, nobody gives a crap if you have a "high end" (not) pc. And for the record, your story has changed at least 5 times. First you were just going to have dx10, then you decided you were getting it with vista, and your video card would run it fine, and FINALLY you've come to the realization that you need a geforce 8000 series card.

I run it at an average of 60 fps at 1024x768, all settings enabled, with 1gb of pc3200 memory, a radeon 9600 pro, and an athlon 64 3200+. The game does not by any means need new hardware.

... oddly enough, my barely overclocked (500mhz core, 500mhz memory) Radeon 9600 Pro performs better than the x1600xt.

I'm thinking $200 is the lowest it could be. As for launch games, I have a sinking feeling that Twilight Princess is gonna get delayed.