Camel, I am in a unique position right now since I have a nearly identical system as you do (except I only have 1 HD4890). I'm sure you know that this setup can play pretty much anything out there with most of the goodies turned on and maybe a little AA. However, right now is a terrible time to purchase new parts with a lot of stuff going EOL and many new, expensive products entering the market. If you want to get something, right now, to improve performance, I would start with a solid state drive, if you don't have one. Crucial m4 128GB is a good place to start.
I would then wait for Kepler and IB to come out before doing much else. I'm not saying wait because I am endorsing Kepler or IB, I am saying wait because prices WILL be dramatically effected by the release in their respective sectors.
Kepler's release is going to be hindered by severe shortages, as many sources are reporting. I would snag an HD7870 or maybe even an HD7950 or HD7970 if the prices drop enough. Then you won't be fighting demand markup, which is likely going to occur for a few weeks if supply is going to be as limited as it seems to be. Not to mention there aren't really any blockbuster games coming out until D3 anyway in mid April.