SLI or Faster CPU?

I'm going to splash the cash and get a new PC soon, partly for performance reasons, partly because I want a machine that runs Vista, for compatibility-checking, and for general up-to-dateness, and partly because I want a bigger hard disk.
The thing is, I'm not sure where it's worth spending the money component-wise.
The two areas I'm dithering between are video cards and CPU (I'm happy with a 300gig disk, and with 2gig of RAM surely?)

options are:

Upgrading from Core 2 Duo E6400 to E6700 (£225 difference)

or

GeForce 7600 GT (256MB) to 2x 256MB GeForce 7900 GS - SLI [upg £ 195.00]

Is SLI more trouble than its worth? I could get a 512MB nVIDIA 7950 GT for £130 increase instead.

I think my compilation times and Poser 6 rendering may benefit from the Chip more, but does dual core cause issues, or does it really give better speeds with those programs? Are these chips fast ones?

Do all games work with SLI? Will this machine be l33t enough for Crysis and Spore? *bites fingernails*

Decisions...Decisions... Next bank statement or accountant summary I get will tip me into hitting that pesky 'buy now' button...