If you have the money go with the E6600 but when it comes to an E6400 vs an E6300 just get the E6300 as it's pretty much the same in terms of power.
The E6600 is on a different level becuase of it's extra L2 cache but still it's not much faster. All these C2D will be able to handle GFX cards without a bottleneck. If you really don't want a bottleneck then just an E6300 to 3.0ghz or so and see if you can bottleneck an 8800GTX even lol, it'll be a bit hard.
You could always wait for April 22nd where the E6320 will come out which is the same thing except 4MB cache for the same price now.
E6300 w/ Arctic Freezer Pro 7 @ 3.6ghz 1.4Vcore
Antec 900 Case
EVGA 750i FTW Mobo
4x1GB of G.skill DDR2-800
2 8800GT in SLI
22" Samsung Syncmaster
1 250GB WD and 1 Seagate 7200.10 in RAID 0
Corsair 520HX 3DMark06 Score: 13900