Definitely stick with the single card for now. If you end up feeling the need to up the graphics power later, the 5770's scale quite well, but its not worth dropping down and crossfiring cheaper cards right now- thats not generally any better performance for the price anyway. 5770 is about as good as you'll get right now for the price/performance, and then you have the option of another later with possibly lower prices if you NEED more power.
cool. Thanks for your answers. I think I will go buy it. Any name brand...?