Did the sums - PSU is underpowered (junk was a fair description .. ). So after a bit of reading have sided with the single rail and future proofing arguments.
Have installed a Corsair TX750W (only £15 more than the 650W) and system fails to boot ... does not reach BIOS. At this stage I suspect that there is no power to the CPU.
Power is getting through to the board via the 24-pin ATX power connector - all fans, peripherals power up.
CPU power (is) provided by the detachable 4-pin connector from the 8-pin EPS12V connector (whatever that is) - am using the correct 4-pin (I hope) "P4/12v" (this has two square and two semi-circular 'pins' that match up to the motherboard 4-pin socket). Its the only socket from the PSU which will fit.
With regards to overheating - am using an AMD x2 5400 with the fan from the AMD x2 6000+ so should not be a problem, should certainly be able to boot into BIOS and POST etc..
If the cabling from the PSU is correct .. then I'm wandering if I have damaged the motherboard.
When I first installed the AMD x2 6000+ (I was very, very *&^$% sloppy) and damaged/bent some of the CPU pins - the system still booted and ran albeit not to design - now there's a suprise! i.e. system booted with incorrectly seated CPU. I checked the CPU, realised what a balls up I'd made, straightened out and realigned all the damaged pins and reseated the CPU correctly. Installed Linux. Linux started misbehaving in ways that I know are NOT down to the software along with the 'freezes' that I have described earlier. So I assumed that I had damaged the CPU and replaced it with a x2 5400. This I have installed properly.
So now - CPU is not damaged, PSU is new, powerful enough and a good one, all cards checked and reseated, connections to peripherals double, triple, ... checked. All I can think of is power to CPU?, motherboard? or something really obvious that I have overlooked!! or something really stupid that I have done.