I had a 4870/512 card so I won't ask why you think you need an upgrade. Highly over rated when it first came out and still is. A 1gig version would have been the cats meow but the 1giggers didn't come out til later. If you look around, unless you game at 2500 resolution...... or want to try and "future proof"........ hate that word............. get a 4890 1gig card. All the reviews I've seen the 2gigs only help at extremely high resolutions........ and you only gain a couple of extra FPS in games......... and with games that have great draw distance..... Grid for one.
On the other hand, if you have $250 to spend wait a few weeks and the new DX11 cards will be out. The "5850" will be faster than the 4890 and should only be $50 more.