Yeah, you won't HAVE to shell out again, because the SRD (that big downloable thing with almost all the rules in it) will be updated to reflect the changes they're making to Revised D&D.
That said, I consider these basically replacements when my old PHB gets beaten up beyond playability (it's gettin' close now...). You can do the same -- buy the new ones when your old books get worn out. Othwerise, use the old one, and download the SRD to help adjust to the changes, and ask on here if anything is unclear.
Basically, 3.5e is the same game, but with a bit new terminology and a cleaned-up presentation and a few new rules here and there (nothing MAJOR major, but a lot of things like altered classes, slightly adjusted skills, some new feats and PrC's, etc.)
It's kinda like 3e would be if all the fan complaints merged with in-house skill to create a new presentation.
So nothin' too major, but certainly the hot topic on everyone's minds right now.
