YOu probably want to keep your AGP card...thats my best guess.
As far as I know, SATA-II controller only come integrated with nforce4,i945/955, the new Xpress200(by ati)...
I have yet to see sata-II on nforce3, the reason is that no further development goes in the nf3 since its older tech...
You can look around, yuou might find a SATA-II conrtroller on PCIe 1X...dont know about the price thought.
Finally even if sata-II is nice too have, it doesnt bring much more performance(if any) at the moment. If you really care about future-proofing yoru platform, you should opt for nf4 and PCIe.
Asus P4P800DX, P4C 2.6ghz@3.25ghz, 2X512 OCZ PC4000 3-4-4-8, MSI 6800Ultra stock, 2X30gig Raid0