I just went through a game machine build I am about to build, after researching a lot this is what I chose, I think you will find it a pretty heavy contender.
I have everything here but the gpu and case so it isnt together yet.
x6800 extreme
asus p5k (p35 chipset)
4 x 1 gig crucial ballistix pc8500 (although you dont need to go that high)
evga oc2 8800gtx
x-fi extreme gamer sound card 7.1
inspire 7.1 speaker system
antec trio 650 psu
400 gig seagate sata 7200.10 3.0 gbps
plextar 16x dual layer dvd/rw
thermaltake 8000bws case
zalman 9700 cpu cooler
now it can and will be argued maybe go with a Quad core, I chose not to yet and I will when they are cheaper later to see if there is any advantage although unless you multi task when you game, right now I dont think that there is.
also you may choose to buy a far less expensive chip and overclock it to near 6800 specs, which has been done, and saves you like 700$, but you can also overclock a 6800 to nearly 4 ghz and obviously nothing else right now will go there in the dua and Q lineup anyway.
you pay a premium for the last mile of speed.
also sli hasnt proven to offer any advantages worthy of spending the extra $500 for another card so that being said the need for an sli board goes away and the p35 chipset has an improved memory controller that so far has shown advantages especialy for gaming.
loaded up with 4 gigs of memory should be a nasty gaming rig.
this is what i came up with, so take it with a grain of salt, I thought it through though and thought enough of it to order it and spend the $$.
was around 2500 without speakers and an os.
the Quad core vs the duo core I dont think will really matter one way or the other right now for gaming anyway so maybe you want to look at a Q core, like I said i will wait til after july and see how the price drops are and maybe grab one then to see if I liek it better than the duo core 2 extreme x 6800.
good luck hope this helps you a little anyway.
if a budget comes into play, go with a 6600 and over clock it, get 2 gigs of memory and go with a lower pc rating like 800 or even lower as I hear you can stiull overclock them well and save money there too, and maybe a gts instead of the gtx, that knocks nearly $1000.00 off the build.
Lump