CPU upgrade, motherboard and 8 Gb of RAM and keep your 5870 for now and the SSD, Your Q6600 is bottlenecking your 5870 as it is, I had a Q9650 @ 4Ghz and that was bottlenecking a 5870.
I would get yourself a second hand i7 2600k or i5 2500k, 2600k will be anywhere from £100-£115 or a 2500k from £85-£95. Then buy a new motherboard and RAM. I don't gamble on second hand motherboards unless I buy on here from the members market from the good guys I can trust, so don't buy second hand motherboards from ebay they will most likely have some fault and that's why some are passing them on there and hoping you don't notice the fault.
A second hand CPU either works or doesn't, simple as that really.
RAM I would only buy again from the guys here on the members market if second hand and make sure the RAM has a warranty, most come with life time warranty.
If you take this route you will have a much better system than the one you mention, the games will run nicer and photoshop will fly and be as smooth as silk.