You might want to save a little money for a DirectX10 videocard when it comes around. But if you want to go all out, that's your business. If you're looking for inspiration, I heard Epic is showing UT2007 on a quad 7800 rig You might also want to check out the hardware review thread, I posted a link there about the the ATI X1900 crossfire (basically a second X1900) and it has some interesting comments on dual videocard set-ups.

personally I wouldn't waste over much over a $1000 on a new PC, all the stuff that's fancy today is obsolete tomorrow. Remember dual Voodoo cards set-ups ? They were ridiculously expensive and pretty much outdated a year later.