S754 diddn't last very long. It'll be harder to find those cpu's in the near future. I belive they've stopped manufacturing cpus for it last year if not earlier. So yeah, s939 will be around for a year or 2. Definately worth the extra cash instead of having to buy another mobo on your next upgrade.
Plus, some mobo's have what's known as "agp-e" which's obviously a agp slot onna mobo that could do pci-e. I doupt you'll get somekindof preformance boost over norm agp, but if you wanna save cash and just get an agp vidcard until you can afford better? there ya go. But obviously, if you can, just go for a pci-e vidcard.
Hmm, onna budget like that eh? yeah, I'm pretty sure you could cram in the s939 mobo, an amd athlon64 cpu (make sure the cpu's s939, ya never know sometimes), 2x512mb of ram (gotta get at least pc3200, higher is good if you can afford it), and a 6600gt (evga is the favorite of the forums hea).
Now how bout the rest? do you need hd's too? there's some guys on here that can throw together some pretty good shopping lists. I'mmat school, so technically I probably shouldn't even be on this forum even though it's comp-related... so newegg n' such is definately off limits.