get another 5770 to run in crossfire
4x on a pcie v2 will give enough bandwidth for a 2nd 5770. Speed maybe reduced by 2-5%, not much.
Instead of going this route I'd get a 5850 for $270 like this one
- and then turn around and sell the 5770 for $100-$120 before the prices drop so the actual cost is only about the same as the cost of getting a second 5770 (mainly since his MOBO only runs the 2nd PCI-e slot at X4 which will effect the performance somewhat)
Or wait and see what comes out the next few months but remember most of the savings in waiting for the 5xxx series prices to drop will also be lost on reselling that 5770 ! (since it's value will also be affected)