I bet you could take it to a computer shop and they would let you borrow a PSU right then and there to test it out. If they do then all you need is to buy a PSU right from them and be good to go!
Also as for temp testling... lol i just slightly feel the heat sink and see if it's hot or not by touch. I personally think it's the PSU though.
Update:
I had trouble getting it to boot up the other day so I just went and brought a PSU in the hope of solving this, it's now not booting up to windows. It tries to start up, struggles like the other day and then stops. No beeps.
Nevermind, eventually it just started working and its been ok for a while. Unfortunately it's just restarted. So not a PSU problem then.
So basically after the new G-card it's been restarting, and upgrading the PSU to a 650W hasn't changed anything.
I've just done diagnostics on the RAM too and it's fine. That only leaves the CPU or the MB. I would have thought if it was either of those it would have happened ages ago, or it would be more consistent. Still restarts for DVD's too. I unchecked the automatic restart box too and it just freezes instead of restarting, no blue screen so that's no help.
I thought I used to be good with computers, just wasted money on a PSU and G-card for nothing.
Well, I've put my old card back in now(didn't even change any software) and things are fine, no restarts, unfortunately it's not a good enough card for what I need.
So it's definately the new card that was the issue. After checking everything on the comp has been updated and trying so many different drivers and checking for overheating and everything I can only presume the card is faulty and will send it back. Thanks for the help guys.