the PCI-E was launched after the X600 and X300, therfore will take time to come, also, there is as well only Intel that made its chipset on PCI-E slots, still there are nVidia, VIA, SiS, ALi to come into market.
you'll have to wait at least 2 months.
Intel Core2Quad 2.66Ghz 12MB L2-Cache
4096MB DDR Kingston Hyper-X
XFX 9600GT AlphaDog
Seagate 2 X 320GB SATA2
LG L246WH Flatron 24"