If you don't really need a computer right now and you're already thinking about nehalem, then I'd suggest that you at least wait for detailed benchmarks on that and deneb before you make any purchases.
nehalem beats deneb by a long shot. the 2.66ghtz nehalem beat the qx9770 at 4 ghtz in a synthetic benchmark. deneb with be the same as the core 2 duos most likely slightly better. and they are not release till nov where as high performance nehalem is released in late sept early oct. any who. i just want to see what i can get for 2500 right now. btw now pheriperals needed just the comp utter. XD
I like the DFI Lan Party X38-T2R. X48 chips are basically higher binned X38 chips, so pretty much everything that X48 chips do, X38 chips can do just as well.
Maybe get Visiontek cards if possible, they offer lifetime warranty on them. Other than that, I don't think you'll be hitting 5Ghz to be quite honest (You'll be hitting CPU limitations, possibly thermal or the very chip), you might want to get a Thermalright Ultra Extreme 120 for the job instead of the Xigmatek.
y not consider starting off with 1 4870x2. They perform VERY well and for the time being 1 might be enough for u. Once u want more frames or the performance isnt what u expected (doubt it) then u can always add a second. That way ur not just wasting money...kno what i mean.
Just see if 1 is good for the time being.
Also id drop the ASCENT ram and go with the regular XP2 mushkin ram. It gets much higher ratings