|"...and a Roswell 750w Power Supply"|
It's Rosewill which is Newegg's "house brand". Rosewill doesn't manufacturer PSU's, they're a reseller. Is this the one you bought?
You could have done a lot better for a lot less money. PSU's with multiple +12v rails should be avoided. Here's a better 650W unit for a lot less money:
And here's some useful info about PSU's:
"so it could just be too much power..."
No, that has nothing to do with it. You could have a 5000W PSU but if your system only requires 400W, that's all the PSU will produce. See the above link.
"what about the 250w?"
Not gonna get it done. If it works at all, it will become overloaded & possibly fry your new video card or worse. Does the old PSU even have the necessary plug(s) for the new card? Don't chance it...pull the new card until you get the PSU replaced.