Reading this:http://www.tomshardware.com/reviews/geforce-310-5970,2491-7.htmlI don’t recommend upgrading your graphics card unless the replacement card is at least three tiers higher. Otherwise, the upgrade is somewhat parallel and you may not notice a worthwhile difference in performance.
Your 4890 is on tier 5......using THG's rule of "3" leaves you with 3 choices
Tier 1 = HD 5970 (4 tiers up) - $625
Tier 2 = GTX 295 / HD 4870 X2 (3 tiers up) - $465
Tier 3 = HD 5870 (2 tiers up) - $410
Again, he says "may not notice......so 2 tiers might be worthy enough if you really wanna upgrade.
There's also a 2nd 4890 as a choice ....http://www.tomshardware.com/reviews/geforce-310-5970,2491-6.html
and at $184, seems a very sound option tho in that board Xfire would be at 8 x 8.http://www.tomshardware.com/reviews/geforce-310-5970,2491-6.htmla couple Radeon HD 4890s will easily beat a single Radeon HD 5870 in the titles that matter today (perhaps this will change when DirectX 11 software becomes more pervasive). From a raw price/performance standpoint, this makes the Radeon HD 5870 a hard sell
But if I was you, I'd wait till post XMas when the non reference designs are in full swing as I expect those will boost things 15-20%. Ina ddition, hopefully, chip yields will get better and prices will drop.....and nVidia may have something out which also could help drive down prices.