I dont see what the problem is -- just get a socket 939 mobo with PCIe. Also, $299 7800GS is NOT a good deal. If 24 pipe 7900GT comes at $299, AGP users will feel largely ripped off (more so than they do now). For the majority of them who are gamers, 7800GS will either not be fast enough anyways, or their processor will be Intel P4 or they wont have an optimal configuration for gaming (i.e 2x512mb ram, XP3000+, etc.). Paying $300 for a card with bad resale value in the future, performance barely faster than from April 2004 period, and not even being a high-end card in 2006 is ridiculous imo.
Either way, anyone who wants Vista, 64-bit, and be able to play Oblivion and UT2007, should upgrade to PCIe sooner than later.