Of course you can find old cards for crossfire. I'm sure the HD5770 will still be available new in a year and past that you can always ebay. I'm sure it will be much cheaper as well.
An HD5770 and crossfiring when necessary is a perfect choice for a 1680x1050 monitor. Advising people to buy video cards more powerful(and twice as expensive) than what they need isn't always necessary guys.
Thank you! This is what I was thinking exactly. It's so easy to just get the next step up but after a while your $1000 build turns into a $1500 build and it's overkill. But thanks everyone for your input - even if I didn't take your advice, it has helped me choose between the two cards!