The GPU isn't top of the range and I don't think it needs its own power supply. I might be wrong. A 520 Watt PSU should more than able to cope. If you can afford the 700W psu then buy that as you might upgrade your GPU at a later date.
It is false economy to buy a cheapo PSU and you do not get much cheaper than £9.99. I am not saying it will definitely fail but if it did it would likely take out your motherboard etc with it, so could cost you a lot more to replace than if you had put a reasonable PSU in in the first place.
The 8600 would not need much more than 350/400W so 500W would be ample and will give you a bit more upgrade room.
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