It's hard to advise without knowing more about the issue. Is yours a hypothetical question, eg. for an essay, or an intended system? If the latter then do you have a budget? Do you know what apps you're going to use? Will they make good use of threads or do they need a beefy GPU? Etc, etc.
I'd recommend a i7-3930 if a high thread count helps, and if the need and budget make it worthwhile, or an Ivy Bridge when they come out. If your requirements favour GPU processing then you can get a lower spec CPU and put the money towards a better graphics card.