Why not just build two desktop machines, one on 680i /SLI for gaming and another on P35/X38/X48 for CPU-intensive multitasking? This will cost less and will be more effective overall.

Paying 1500$ for 1 CPU is completely insane, if you ask me.
I paid 900$/each for Opterons 265 3 years ago and 900$ /each for Xeons E5320 an year ago, and i would never do this again unless i will be absolutely pressed to do so.
This super-expensive stuff just do not justify the cost. Unless you are Bill Gates, lots of frustration is imminent. Soon you realize you could get 300-400$ CPU which, overclocked, could perform almost just as good.
Then again, at the moment, Intel's dual platform is severely crippled by memory subsystem, while AMD... they just don't have CPUs at all