2 PSUs in 1 Computer

No, running 5770 on a 250W will risk damaging other components. You wouldn't want those components to die off would you?

In fact, even running your system with a 5670 on an old 250W will put your hardware in risk on full load. Change your power supply before you change other parts.

Yes, its doable. The issue is to make sure the two PSUs have a common ground.

I'd take an unused 4-pin molex connector from each psu, and cut one black (ground) wire near the connector. Then strip a bare end on each and twist them tightly together (solder would be best), cover with black electrical tape.

I'd also feel better if the external power supply stayed in cnotact with a metal part of the case, but its really not necessary. The fact that both have three wire plugs should ensure a common AC "ground".

The PCI-e power cable to the 5770 would come from your "external" psu, the much smaller amount of power coming through the motherboard from your "internal" psu.

It's a shame because you're maybe $50 away from the safe, sure solution - a good psu.

Here is the downside:
http://www.tomshardware.co.uk/forum/279908-12-motherboa...

Yup, that's exactly what can happen without a common DC ground.

A PSU will deliver ~12V *differential* between the two wires called +12V and ground. A second PSU will do the same. As long as the differential remains 12V, "nothing" else matters.

However, if you measure the voltage between PSU#1's ground and PSU#2's +12V wire, you don't get +12V . . . and the reading will fluctuate. Because "0" is not the same for both PSUs.

That gets . . . well, more than confusing for the poor mobo and its friends.

Would I do it? No. (Though I did it a *lot* using old recoverd PSUs to power both the rails and the control circuitry for HO railroads.)

And if my son burned up his PC and maybe the house trying to sneak a fast one past me so he could have a 5770 instead of a 5670 . . . I'd do a lot more than fan his ass.

Thanks guys i guess i'll figure it out possibly sell some my paintball gun or something to do it the safeway I was wondering if it was risky and according to you guys it is.

It's like defusing a bomb - its possible, but *you* shouldn't do it.