According to the December issue of PC Gamer, the ASUS V9999 is actually a 12-pipe 6800 gpu that is clocked at the same speeds as the 6800 GT (which has a 16-pipe gpu and is A LOT faster than the V9999). The reviewer said the V9999 is misleadingly labelled because it does NOT have a 16-pipe gpu, and not worth the money because it's basically just an overclocked 6800 (which you can buy for around $270 US).
If you prefer nVidia, I recommend buying a real 6800 GT since, from a price/performace perspective, it's the best card nVidia makes. Otherwise, you can just buy a 6800 and overclock it yourself, although I don't know if it's possible to flash a 6800 to unlock the extra 4 pipes to make it a GT.
The 6800 GT pretty much kicks a regular X800 Pro's ass (because the x800 Pro has only 12 pipes enabled). However, if you can find an x800 Pro VIVO and sucessfully unlock the extra 4 pipes, then it's a whole different story. Also, some people who own both cards say the X800 has better image quality, but that seems to be rather subjective.
If you really don't care about the fastest benchmark scores, then it doesn't matter. Either card is so much faster than what you're currently using you'll be very happy. However, it doesn't seem like it's worth the extra money for the V9999 since you can buy a regular 6800 for much less and overclock it yourself.
) so.. which one is better ? please reply 