I am upgrading from a 5770, due to the impending (mythical?) Steam Summer Sale, and some specific titles (Arkham City, Skyrim) that I wish to play on max settings at 1920 x 1080. I am between an MSI Twin Frozer 7850 or 7950.
Let me describe what I am looking for: I am hoping to get a card which will allow me to play max settings for most games at 1920x1080 for the next 3 years. There is a possibility I might pick up a Catleap or the equivalent at some point (and thus the slightly higher resolution), but this is by no means a guarantee (I have a Dell U2311 I am quite happy with right now). Specific games I am looking forward to include Bioshock Infinite, Alien: Colonial Marines, and The Elder Scrolls MMO. I currently play LA Noire, SWTOR, Deus Ex.
I have no troubles overclocking, and will run this paired with a 955 BE @ 3.8 Ghz. The price difference (both MSI Twin Frozr I am looking at) is about $110. I can spend the extra if I need to, but only if its not going to be a waste with the performance I am hoping to get. I guess I am a bit skittish with the 8000 series around the corner, and I am stuck between "buy the fine one now, and upgrade sooner" or "buy the more expensive one now, and pray it lasts me as long as I'd like."
If you want your rig to go on for another 3 years with the same GPU then you ought to go for the last and latest available GPU from either Nvidia or AMD. Plus you need to have room to enable SLI or CF later on.
With Catleap, you'll need HD 7950 at least, and even then it's uncertain if it will suffice in 2-3 years. I currently run HD 7850 and I have Shimian and it runs most games very good with maximum settings (except AA), but I'm planning to CF down the road.
The 7850 is a great card, but given your needs, I'd think you should only get one if you plan to crossfire it as an intermediate upgrade in, say, 18 months. The 7950, especially overclocked, is beastly. And the upgrade from a 7850 is pretty linear in the price, too--even better than linear. Assuming the 7950 costs a bit less than 50% more, look at what you get: 75% more shaders (1792 vs 1024), 75% more texture units (112 vs 64), and 50% wider memory bus with 50% more memory. (It also uses about 50% more power, incidentally). Those differences are borne out in the benchmarks, which are about 20-50% better on a 7950: http://www.anandtech.com/bench/Product/549?vs=550 Plus I agree that the 3 GB of VRAM is important if you're consider higher than 1080p gaming.
And the 7950 is a great card anyway. It overclocks like a beast (so does the 7850, of course). I'd go for that.