Yes it's possible. Basically it would generally perform between two 4890s and two 4850s in CF. The 4890 will not down clock itself to match the 4850
, or at least it shouldn't
. There are a few instances where it may perform like two 4850s rather than between the 4890s and 4850s, but that's a scaling issue that seems to lessen with each driver release. There are also some quirks that arise with using two different cards in CF. Also, if the 4850 is a 512MB card then it is best to get a 4870 512MB. Spending extra money on a 1GB card would be wasted because the drivers would only treat the 4870/4890 as a 512MB card to match the frame buffer on the 4850.
As for the PSU, yeah get a good 600W or above unit.