Well I have a gaming machine
unfortunately couldn't afford quad core
but I got ATI X1950 512MB at the mo
best on the market they say
think you could crossfire them to :
Motherboard, don't really no, it all depends really
I had to buy £100 psu with three 12v rails to power it
I had trouble with a few mobos not working with it
but picked one off the site, fairly nice can take 8GB + Dual Channel mem
It all depends on you budget the exact stuff you get though...