Locrieth :
looking at this link (page 22):
http://shop.itema-pg.com/userfiles/editor/file/hp8200sff.pdf
it lists all the various form factor 8200 elites and it shows the PCI express x16 can only give a maximum of 25W, I'm guessing that means it won't work? I've just had an 8200 SFF being scrapped out at work and was thinking of doing the same, but wasn't sure if it would be compatible. Has anyone actually got this setup working, before I Blow £150?
You're reading the specs sheet right.
The HP 8200 SFF was designed to be as an office PC and hence the reason why PCI-E slot can carry only 25W in it.
One of the GPUs you'd be looking in it would be Radeon HD 5450.
pcpp:
http://uk.pcpartpicker.com/product/9tDwrH/xfx-video-card-hd545xzch2
GPU has TDP of 19W and it's low profile with passive heatsink.
When i bought my AMD build back in 2011, it also came with Radeon HD 5450 GPU. I managed to game on HD 5450, it wasn't pretty but it worked. Currently, got my HD 5450 at the drawer since i upgraded my AMD's GPU to HD 7770. But enough of that.
When looking the specs sheet, page 17, then it can be seen that SFF is supposed to support Radeon HD 6450 GPU. HD 6450 has TDP of 27W.
pcpp:
http://uk.pcpartpicker.com/product/XRmLrH/sapphire-video-card-100322l
Here's comparison between HD 5450 and HD6450: http://gpu.userbenchmark.com/Compare/AMD-Radeon-HD-5450-vs-AMD-Radeon-HD-6450/m7719vsm7821