Some "benchmarks" (no one knows if they were actually true or made up...prob made up) came out a while back and an article on the r600
fade2balck posted a couple of days ago. As for the 8900gtx, this
was posted a while back.
r600 (x2900xtx): 512-bit memory interface, 700 million transistors, gddr3 (for the x2900xt version) and gddr4, 1gb memory bandwidth (reference configuration), and two-slotted (i guess that means its like the 7900gx2 with two gpus???)
8900gtx: 80nm processor, 128 shaders, core clock: 700 mhz, Memory Clock: 2200 mhz, 384-bit memory interface, gddr4, 768mb memory bandwidth.
I guess with more memory interface and memory bandwidth, the r600 looks like it will be better; but for what price i wonder? Still don't know the clock frequencies and number of shaders though, your just going to have to wait and see until the cards are actually released (at least to creditable techie sites to give benchmarks) to actually see which one is actually better.
My personal opinion: I have my eyes set on the r600.