From what I've heard, the 6800 and 6600 are so similar, that the 6600 is the obvious choice, because it's so much cheaper. Ghost says the 5500 is even more worth the value, so maybe go with that.
But I'm not completely sure, so let someone else roger that.
My reply to all you show-offs out there:
NVIDIA GeForce4 MX 4000 PCI@275
256mb PC133 SDRAM@100 3-3-3-6 /x2
Hansol 17\" CRT 710D Monitor
VIA AC\'97 Audio Controller
Windows XP Professional
HITACHI DVD-ROM GD-7500
LG CD-RW CED-8120B