8800GTX is a ripoff.  The GTS only runs a tad bit slower but costs so much less.  Even if you're going for a high end machine, get the GTS.

VRAM is seriously not used much, even now.  I have a 256MB video card and I expect that the VRAM will not be a problem for quite a few years at least.  I think Quake IV is the _only_ game that actually makes use of 512MB VRAM, and that's only on ultimately supreme mega high settings.
Games such as Oblivion and FEAR do not need the massive amounts of VRAM.  I highly doubt Crysis will ever either (it's been tested running well on high settings on a 256MB card).