Really hard to say... I'm not that familiar with HD4850/4870. but looking at raw benchmarks, below is a comparison of the 4870 vs the 6850:
http://www.anandtech.com/bench/Product/304?vs=291
and a comparison of the 6850CF compared to Nvidia's GTX 580 (best of 5** series)
as you can see, 6850 CF is definately on par with 580 preformance, while a 4870 gets about 2/3 of a 6850's performance, so my guess is performance wise you're probably close to a Radeon HD 6870. if that's the case, you'll see plenty of people recommending the 6870 for gaming on a budget. The caveat is of course you won't be maxing out the newest and greatest games.
BUT, and this is an important BUT, the 48** series are much older tech, as such, you're more constrained on the VRM front, hardware's also not optimized for newer engines such as DirectX 11. if I were you I'd start saving up now, and upgrade either to AMD seaislands (HD 8*** series), or Nvidia's GK110 (GTX7** series) sometime in Q4 2012 or Q1/2 of next year
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Edit: BTW, I hate telling people their system's already dead because you're obviously still using it without too many issues. and generally, if you're on a budget (which almost everyone are), the best strategy is to make your current system last as long as possible before the next upgrade.