ATM, The Radeon Xpress 200 series 939 chipsets and XP-120 + AMD 64 3000+ along with either 1gb or 2gb of OCZ PC-4400 (or 4200)
cranking the FSB to 250+ (and outpacing a stock FX55 in some cases

)
If i had a choice... i'd go with the X800XT
But ATM, i'm still waiting for availability of these parts... and by that time.. the X900 series will be out.......