I've got the Zalman 850 W Power supply and have been wondering the same question. It is a VERY nice power supply, Zalman really did a great job on it, but I know that the power req. of the 4870 and the 4870X2 will be quite high. The Zalman 850 W is 9800GX2 "Quad SlI" certified, so it certainly can handle alot. Hopefully it will maintain Tri Xfire. Anandtech just did a review on the 850, which ended up being a quite favorable conclusion. Also, Anandtech just reviewed some low,mid,and high range Power supplys and the Zalman 850 is considered a great buy for the high end users. I have seen alot of Zalman power supply haters on these forums and I have no idea why. Try the dam thing before you bash it, I love mine. http://www.anandtech.com/casecoolingpsus/showdoc.aspx?i=3359&p=4
This is the link to one article, I am having trouble getting the other one to work.