1gig 2900xt Crossfire problems

Hey. Bought to 2900xt's and i just cant seem to get my crossfire working properly. I get better proformance out of a single card then i go ran both of them stock or overclocked. I've switched them around from primary to secondary, reinstalled drivers probably a dozen times installed hotfix's (the ones i could find) and i have even done a system format to try and resolve the issue, all drivers for everything on my system is up to date. Help please cause i dont know what else to try. Here is my system

AMD 6400+
4gig Corsair XMS2 DHX 4-4-4-12
MSI K9A Platnium
1200W Thermaltake PSU
2x HIS 1gig 2900xt's
3x 320gig Seagate HD's
Windows XP SP2 up to date with Automatic updates

Thanks for your time.