From the specs I'm seeing for that unit it should have slightly over 300w on the +12v rails. If that's the case it should actually be able to handle the GTS 250 which averages 105w during normal gaming.
Yeah, the person i brought it off on ebay said it wouldn't power it but from what i can see from him he does not know the differences from a sempron 140 and a dual core CPU
I dont know if this will work but its worth a shot so i got 6 pin PCI express to dual 4 pin molex in hope that i can plug that into the molex coming out of the PSU and plug the 6pin into the other cable and into the GPU.
Hope this works