CPU Confusion

[Professor S]

I'm finished saving for a ring, and am now in the process of saving for my future PC. Due to the fact that my current PC is slowly falling to little itty bitty pieces, my time frame has been moved up and I will not be able to wait for Vista (which is probably a smart move) nor will I be able to spend upwards of $4,000.

So here is the build I as thinking of:

CPU - Core 2 Duo 6700
Memory - 2 Gig Corsair "Xtreme", upgradeable to 4
Video - SLI: x2 nVidia 7900 256mb
OS - Windows XP Professional (its more stable)
Price - About $2,000 with other peripherals from Cyberpower + a 3 year warrantee and customer service.

The Core 2 Duo Extreme is way too expensive for not inceasing performace by very much over the 6700. BUT... the specs on the 6700 and 6600 are very similar, is there a big speed difference between the 2?

When having an SLI, how important is it to have 2 beastly vid cards? Would upgrading or downgrading affect the performance that much?

DirectX10 is only going to be available with Vista. Will that crush my gaming dreams when Vista finally comes out?