AMD Quad FX systems aren't available yet in quantity but the Inq did some digging and found out that you'll have to shelve out at least $2,999 to get your hands on a Quad FX based PC.
Vigor Gaming calls its system Quadfather and the cheapest one with two FX70s at two times 2.6GHz costs $2999 including a free shipping and has 4x512MB Corsair memory, two 250GB drives in RAID 0, Geforce 8800 GTX and 1100 W Tagan PSU. The company even offers a free Vista business upgrade.
We had some fun and tried to make an ultimate gaming system with 4GB Corsair memory and two FX74 and two Geforce 8800 GTX and we ended up at close to six thousand dollars. That is a lot of money but it cannot get much faster than that.
Unless you have a lot of money to waste there's not really a point in buying such an expensive computer IMO.