both will not likely last 3 years. the 5770 is the faster single card although you can get some highly OC'd gts450's that match and sometimes beat the speed of a 5770, but they cost more too. If you want it to be futureproof for 3 years you really need to look into a higher end card like a 6950.
hd 6950 2gb seems like a attractive choice for now. there are a few reasons for this.
-is cheaper than gtx 570
-it is unlockable to hd 6970 with bios flash(but i won't do it anyway even if i have 1)
-assuming that AMD already totally solved its driver support issue for crossfire,
its 2 gpu configurations is far superior to that of gtx 560 ti SLI.
-Uses far less power in crossfire than GTX 570 SLI
buying two of these and crossfire will save you lots of money than buying 1 hd 6990. While not far behind in terms of performance.
While me myself prefer a gtx 570.. but hd 6950 2gb is so good it that i can't deny it is a better choice over the 570..
"Future proof" is a myth, there is only "future resistant." If you're willing to lower settings enough, an older HD3850 can still play current games, but I don't think that's the point. Will your budget allow you to get a GTX460? Even the 768MB version is stronger than the HD5770 pretty much across the board: http://www.anandtech.com/bench/Product/296?vs=315