I have an FX-8350 that I've overclocked to ~4.2 GHz. I haven't really run into any notable performance issues yet. My RAM has been bumped up to 1866 MHz as well (which, I dunno, maybe makes EFB copies a little faster or something).

If it matters, my video card is a GTX 570 2.5GB (don't be fooled by the 2.5GB, it is a real card!).