Of course, it always comes down to price.
I did a search at newegg (love their search interface) for all Gigabyte pci-e video cards priced from $100 to $200.
I got
1 6600 - $108
3 X700PRO - $108, $152, $174
2 6600GT - $169, $196
1 X800 - $196 - $30 MIR = $166
I put them into a compare format (table) and found only one had a 256-bit memory interface. It was the x800. Only one had 12 pipes; it was the x800. Only 2 had 256MB memory, one was the x800 and the other an x700pro. Only one had OpenGL 2.0, the x800. Oddly, only two had fans, an x700pro and a 6600gt. All the rest had passive cooling.
Very important was only one has a rebate-- the x800.
Cons:
But for the 6600, the x800 had the slowest (392MHz and 700MHz) listed core AND memory clocks respectively.
The only difference between the X800 and the X800PRO is the PRO comes with 475MHz and 900MHz core and memory clocks respectively. If the X800 np could OC you'd have the PRO. Of course, the PRO model may OC, too.
Personal hope: Although this x800 is noted as either a poor or fair overclocker, I have excellent case cooling so I'm hoping for a little OC. With 12 pipes that may interpret into a nice little speed jump.