Personally I would NOT be chasing second hand Q9550 and DDR2 for an old PC.
Quite simply, 8gb of DDR2 second hand is around the same price as 8gb of superior DDR3.
A Q9550 is only a bit better than what you have if you use 4 cores, my E8500 overclocked was a better gaming experience on most games in comparison to the Q9550, both player World Of Tanks, Planetside 2, Bioshock Infinite, Crysis 3, BF3 etc.
I had planned on using my Q9550 system until the end of the year, but an unfortunate accident ruined my motherboard. It would have done fine until Skylake was released.
But I ended up going Z97. Skylake will offer around 15% more performance some sites claim. But a Skylake system is going to cost more than 15% above what is currently chosen, you could argue that it will be very likely not a cost effective upgrade for anyone on Z87/Z97, because as we have seen, the two generations before those have had no reason to upgrade.
As such I have no issues with recommending you just going Z97 now, and would say a Haswell i5 with a Z97 motherboard and 8GB of DDR3 and a 290X GPU will be a great upgrade, which will do you until DDR4 prices drop and we see improvements on Skylake.
Current prices are not going to fluctuate much,
))
I'd be aiming for under £20
