For that budget your going to want to go with an H57 board (sub $100). I believe Asus has one that will run two vid cards...the EVO but it's not cheap. Most likely go for a single card set up..that and a psu that's going to push two decent cards will surely push you over that budget.
you can easily make it about 700 bucks, with the parts there is would be about 730 as is, and that depends on what motherboard you purchased. Could save a little more cash is you downgraded HD etc. without losing and CPU/graphics power if you so wanted.