5770 in crossfire would be MUCH better. I didn't even know 5830's still existed, at least not in Australia, they were a flop. If your going to get a better single card, get a gtx460. faster and uses less power than 5830. If I were you i would get the 5770 for xfire, or another cheap 5750 for xfire. That should keep you going for a while. Keep in mind there are a couple games that dont officially support xfire, so look at the games you want to play and do some research before going the crossfire route.
+1 on the Crossfire 5770s. The HD 5830s really sucked just like the GTX 465 did. Both were basically flagship GPUs that were heavily cut down and they both underperformed, and weren't worth the heat, power consumption and price.