Personally I would step up from a 5830 and get a 5850, the 5770's in crossfire cost more than that option so it should be within your budget.
Your resolution will matter here and if you dont have a monitor capable of 1920x1080 then just get the 5830.
Crossfire and SLI while alot better than it used to be is still hit and miss with the performance dependant on Game and Driver support.
What i suggest is you take a look at this review http://www.techpowerup.com/reviews/MSI/HD_5830_Twin_Fro...
Then decide based on the review results at your resolution and the games you play. I would have thought a 5830 would have been the most you really need but its your call at the end of the day.
This is the one I was planning to get. As far as Memory clock and core clock, they are the same. Although the one I want to get is almost $260.
I'll do some more research, and try to find out which card is better for overclocking.