gear999 :
Neither. FX 8320 is fine, the rest (8350, 9370 and 9590) are just factory overclocked versions of the 8320.
The 8320 is a factory underclocked and lower binned 8350.
@OP,
The 8350 is pretty much where it's at, the 9000 series are much higher binned, much hotter and much more power hungry chips. Personally I don't think they're worth the extra cost even now that they've come down in price - When you consider that you're about 50 quid away from an i7 at their current price tag.
They require a decent board to pump that many volts through and a decent cooler to dispell the heat (There is a boxed water-cooler for one of them now I believe). You may also need a decent power supply since their stock TDP is 220w.
If you're on a budget then the 8350 is a very affordable chip right now. Keep in mind that it's about 2 generations old and AMD haven't been keeping up in the higher end CPU market, so technically it's their current high tear CPU line up. Meanwhile Intel have had an extra 2 generations making the 4000 series and indeed - New Devils Canyon version of them, a much more logical choice for a current build.
I'm personally sticking with my 8350 because I bought it around the time it was released (ala when it made sense) and still love it. But currently, for you, a latest i5 or even i3 might be just the ticket.