5770s are hella cheap but make sure you get one that has quadfire capability, but
r-sky is right, purchasing one of those gpus would be very shortlived at this point in time, with all the new changes in graphic design that are happening even the 5770 would see its demise within the next 2 years, i recommend a 5850, you can get the asus direct CU version for 200 on newegg and that one will overclock past a 5870 performance level giving you much more performance than even 2 5770s
IMO crossfire and SLI are for people who either, want 2 top end cards and have lots of money to get best possible performance, or someone who already has one of the cards and is wanting more performance and gets another. If you are starting off buying a card, get the fastest single card you can afford and get another one of those to crossfire at a later date. 2 5670's or 2 5750's will cost more than 1 5770. for the price of 2 5750's you could get a gtx460 which is better than all those cards. Also, crossfire is for people who like to tinker with settings as its not always as simple as "plug and play" to get crossfire working properly.
If I had to choose between the three I would go for the 5770 just because it should last longer than the other two, and you could eventually pick up a second 5770 for pretty cheap later on down the road. Crossfired 5770's on same games compare with the 5850 and even the 5870 on the occasion.
EDIT- Reviews also show it does better than crossfired 5670's but the 5750 can get close to the same as 5770's. I still would pick the 5770 though if thats the top of your budget.