There are some general 5900 ratings, but they differ by company -
5900XT/SE - Usually runs 400/700 (2.8ns RAM)..BUT...RAM timings are more agressive to compensate
5900nu - Can run from 400/700 to 400/805 depending on manufacturer. As a rule 800 means 2.2ns RAM & lower means 2.8ns RAM.
5900 U - 256Mb & 450/900 clock, 2.2ns RAM
5950 - 256Mb & 475/950 clock 2.2ns RAM
As a rule if you see a 5900nu w/ an 800 (or higher) RAM clock you'll be getting 2.2ns RAM & this card will most likely hit Ultra or better speeds. If you see a 700mhz rating chances are that you won't get as high an OC on the RAM, the CORE will still clock as high as any other (luck of the draw, mine won't go above 465 on stock cooling).
NewEgg is out of the BFG 5900's but here's the link -
BFG 5900
I've heard of a place called GameVE (??) that is selling it for $161 - but I know nothing about them.