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Q9550 still fairly good but not really cost effective at the prices they demand these days - worth doing IMO if you can source one cheapish though.
As cheap as possible
A second hand 2500K would be worth considering, they may be a few years old but if you can get a reasonable OC on it they are still very good CPUs.
As you like playing KSP it would be worth looking at performance in that. My understanding is that Intel generally has the upper hand over AMD.

I upgraded from my Q9550 at the end of 2013. An excellent processor that served me well and overclocked fantastically (4Ghz on air), but it was starting to get bottlenecked in modern games and was starting to hurt my fps. For the price (for some reason Q9550s and associated hardware can hold their value well), I would try some of the suggestions mentioned above![]()
How about you wait until the new Elite game is released??
Supposedly it is meant to support Mantle too:
http://videocardz.com/51018/exclusive-upcoming-games-support-mantle
So if you have a GCN based graphics card,an FX CPU might do well.
However,it is still worth waiting so we can see how the engine performs.
What resolutions were you gaming at?

1920.
Assassin's Creed 3 was the first game where I couldn't maximise everything - a very CPU demanding game, as was Black Flag. I noticed an improvement across all modern titles though, in loading times as well as fps. I was hoping to keep my Q9550 a little longer, but given the price I got for the hardward and deal I got on OcUK, I just couldn't resist the upgrade![]()
Yeah... at that res probably right, but on a 24" at 1920x1200 I can't really notice any improvement on running AA above x2 really. Especially in FPSs... I tend to max everything else though.Until recently I had a Q9550, it was a great chip that clocked easily to 3.4GHz and probably beyond in a decent P45 board.
If you want 8 threads then I'd recommend a Haswell build (i7 or Xeon E3) over anything AMD have to offer.