The Asus 6800 GT 128 meg uses the full-speed 6800GT GPU, the same processor as in the 6800 Ultra but slower than the ultra. The 128meg GTs use 700mhz memory and are cheaper than the 256 meg GTs which use 1000mhz memory. ASUS is making their cards non-standard and confusing things

Reference designs:
6800 - 12 pipes, slower gpu, 700mhz memory (128 or 256 meg)
6800 GT - 16 pipes, 350 mhz gpu, 1000mhz memory (256 meg)
6800 Ultra -16 pipes, 400mhz gpu, 1100 mhz memory (IIRC), (256 meg)
ASUS designs:
ASUS 6800 GE - 12 pipes, slower GPU, 1000mhz memory (128 or 256 meg, dunno)
Asus 6800 GT 128 meg - 16 pipes, 250 mhz gpu, 700 mhz memory (128 meg)
BFG makes "OC" models that come from the factory running at higher speeds than the reference designs. They may have custom heatsinks that helps them get this higher speed. You'd pay a bit more but they run at what are typically considered overclocked speeds for those cards straight out of the box, and they still have a full warranty at those higher speeds.
Clear as mud now? The 6800 GT 128 meg you quoted is probably the ASUS one which is why it's a bit cheaper than the other 6800GTs out there. A full-up 6800GT ought to cost at least $375-$400.
I personally looked at the prices and performance and got myself a 6800GT with 256 meg of memory, one of the "reference" designs that are the same from all card manufacturers except ASUS. These cards have a really good chance (apparently over 80%) of being able to be overclocked to 6800 Ultra speeds with zero modifications.