Just a note - if the purpose of this machine is single-core compatibility, I'd steer away from the 939 dual-cores. They have the timing bug, which can break games/applications that will otherwise work on SMP machines, and the AMD "Optimizer" application (if its still distributed) does not provide a universal fix. That said, that Opteron should be fine if you add more RAM. My P4EE was/is fine in Vista with 2GB+ of RAM, and the Opteron will be similar (if not faster) in performance.
On the graphics card - aside from improving performance in games, and adding SM3.0 support (not that uh, GeForce 7 will do a stellar job with "heavy" SM3.0 games), there'd be no benefit to the GeForce 7. They won't do anything for web browsing/web content (nor will the X800). So I'd probably pass on such an upgrade unless you need higher gaming performance.
Something to consider though - there are PCIe-based 939 boards, and you could throw the Opteron, 2GB+ of RAM, and a GeForce 8600 (or similar) in such a board and have an overall very good performing machine that would also handle the web/multimedia much better.