ASUS - 1000MHz OC: http://www.newegg.com/Product/Product.aspx?Item=N82E16814121650
Sad though, my favorite graphics cards are manufactured by Gigabyte; buy I wouldn't buy their 7850 - not a Windforce X3 model (I always get massive overclocks out of my Gigabyte cards that are Windforce)
Regardless, something to keep in mind if you want to overclock any graphics card, however, is that XFX's Double D, Gigabyte's Windforce X3, ASUS DirectCU II, Sapphire's VaporX, and MSI's Twin FrozrII are the best coolers on the market and make sure if you get any version of a card they have those (for example, DO NOT buy any XFX "core" edition card).
As for crossfire with a 600W PSU? Unless you have the best rated 600W PSU on the market I doubt your system would ever be stable at load with two GPUs on a PSU with that low of a watt rating.