As long as everything else works, you should be fine. Keep in mind, you're likely going to need a new PSU as well. The 5830s can now be found on ebay for ~$150 each and if overclocked properly can bring 300MH/s if pushed to the max. The 5850s range from $200-300 on ebay, and can pull in 400MH/s each if pushed to the max. Brand comes into play here because some are much more easily overclocked than others.
Realistically, the 5830 is the most economical option, although they won't get you to 750MH/s.