The GTX 550Ti has one 6 pin connector and draws a max power of 116 watts, while the GTS 250 is essentially a re-badged 9800GTX, has one 6 pin connector and draws 145 watts at max power. It's not a power supply problem, because the 250 works when plugged in. Unless there is a seating problem with the connector, there's no reason it should not boot up. Double check your seating at the PCI-E connector and carefully re-seat it, and FIRMLY. From there, carefully plug in the 6 pin connector, again, FIRMLY. From what you are stating, it seems to me that you have a grounding problem, but from an electrical standpoint, it's more like a short. Try checking the contacts individually under magnification. Hopefully this helps.
Past that, I'm out of ideas.
