It may be 600w on the label but it only has a 480w 12v rail which is pretty rubbish by modern standards. It will do for now though. If you upgrade to a modern cpu/gpu in the future it will need replacing.
You had better remove the link as well because they are a competitor.
Going by the
cpu support list for your motherboard the best cpu's it will take are the Phenom II X4 965 and Phenom II X6 1090T but I am not sure if they will be held back in a AM2+ board. If I recall the main difference between AM2+ and AM3 was DDR2 ram for AM2+ while AM3 used DDR3 Ram and the hyper transport was a bit slower in AM2. How much effect that has I don't know. With that in mind you could try to pick up a second hand Phenom II X4 965 black edition or a X6 1090T but don't pay too much for it. You will need to update the bios to the latest version using your current cpu.
Comparisons between your 6000+ and the two potential upgrades:-
6000+ vs 965 BE
6000+ vs 1090T
As you can see both are considerably better than your 6000+ so would be a upgrade. Like I said though, I don't know if the AM2+ board and DDR2 will hold them back or not.
That would leave some money for a second hand gpu. What gpu do you currently have? Something along the lines of a AMD 7850/7870/7950 should be less than £100 (£65 or less for the 7850, £75 or less for the 7870) or Nvidia's GTX750ti, GTX760 (both £70 or less), I have also seen GTX770's go for £90 or less on the bay. Anything more powerful and that psu would need changing now.
If you wanted to go for a new build using new cpu/motherboard/memory/gpu (The cpu is a big step up from what you have now but there is no future upgrade path) then the best you will get for £250 is this:-
My basket at Overclockers UK:
Total: £250.46
(includes shipping: £10.50)
If you prefer a AMD gpu then you could swap to the R7 360 although I don't know how it compares to the 750ti. Neither of them are going to set the world alight anyway.
My basket at Overclockers UK:

If you wanted to add in a cheap but decent cooler for some overclocking then add this to the basket. Otherwise the 860k comes with a cooler but they can be noisey.
My basket at Overclockers UK:
Total: £22.38
(includes shipping: £7.39)
