It's likely if you add an extra 5770 you'll need to upgrade. But if you just stick with a single card, IE 6870, then you probably won't need too.
Honestly though, your resolution is so small you don't even need a graphics card upgrade. You really don't need to upgrade anything. The only thing I see needed is another 4GB or ram, I hear programming takes up a bit of ram space. (That is that's what my teacher in Comp Prog. tells me about Eclipse)