in your position I'd be tempted to wait/save a little more to get a 4890 or similar to make it a really worthwhile upgrade.
especially when new cards cards come out in about 3 months... and their prices drop.
I have an 8800GT 512 and will be doing similar, But I have to much work on to play much atm so makes not so much sense for me to upgrade quite yet
I think you would want to get an ATI 4870 to 4890 or Nividia 260 maxcore to 275
The 1st options will give a decent upgrade for a little
The 2nd options will make for a great upgrade and last another year or 2 easy
as it seems you don't upgrade too often, do it to a lvl that is going to last you some time.
The 4890 is a lot cheaper than the Nvidia parts and should last you well, that and it OC's beautifully
1.Cooler (most are the same but some like the HIS IceQ are different, but in turn more $$)
2. Company warranty and customer service (this category goes to XFX--which is the one i think you should buy)
3. Packaging and gifts
For whatever reason ATI cards do not perform well in wow compared to nvidia. A 4870 would not be much of an upgrade over the 8800 for wow at resolutions of 1680x1050 or lower. Though this is probalby the only game where that would be the case.
Resolution, how long you want to keep the card, and the rest of your specs would help.
yeah daedalus brought up a very good point and question that end-users often forget to ask themselves. How long you want to keep the card makes a huge difference. are u looking at a card to keep you going for 6 months to a year? okay maybe a 4850 could do that. If you want to keep it longer, get a better card like the 4870
DX9 applications (which include wow, and source games) will not use crossfire in widnowed mode. Keep that in mind before you buy a second card if you like to paly in windowed mode (I know I can't stand wow without windowed mode, which doesnt use my second 4890..).
two 4850's would perform much better than any single core card in games that use crossfire and are not entirely memory starved (for isntance a top of the line 1gig card might creep ahead at 30" native resolutions)
For games like maxed out crysis at very high resolution the crossfire scaling may be close to 100%. For the vast majority of games we would be talking about 60% scaling in most cases.
I pointed out how long you wish to keep this card because we are approaching a new DX release. If you are the kind of person who upgrades every 2 or 3 years you may want to hold out until then with your current card. If you are one to upgrade every 6-10 months, or absolutely need a card now then it doesn't matter.
DirectX is an API (Application Programming Interface) that basically talks to your 3d hardware in your computer and tells it what to "draw". They change it every couple of years to improve graphics and performance. if you want more info, look it up on wiki, lots of useful stuff there