I voted for the AMD X2.
Almost one year ago I had both systems for a couple of days, then donated the X2 machine to my dad; 939 X2 4800+ with 2Gb RAM (ocz platinum rev2), 7900GTX SLI GPUs on DFI Lanparty Nf4 SLI
New machine was X6800, 2Gb DDR2 (corsair), 8800GTX SLI GPUs on ASUS P5N32 SLI SE Deluxe board (worst board I ever had)
I still remember the feeling after I first installed XP on the X6800 machine, it felt slower compared to the X2. I thought it was the drivers so I got the latest updates, BIOS updates, everything, it didn't change.
Right clicking the desktop on the X2 brought out the menu faster than on the X6800 machine. IE opened faster, etc.
I think it has something to do with the integrated memory controller on the X2 that it feels snappier.
Performance wise, X6800 killed the X2 4800. I've tested both with the 8800GTX SLI; I don't remember the numbers, but the X6800 lead was solid.
So; If you want a snappier windows, go for X2.
If you want performance, go C2D.