Most manufacturers recommend a rating of 24A or 25A on the 12V rail(s) for the GTS 250. You have a rating of 29A. It also needs only one 6-pin PCI-E power connector, and you just happen to have only one of them. It should work just fine in your system. The 4850 requires a bit more power than the GTS 250, but should also fall within limits.
I'm not certain it's entirely accurate, but try consulting this chart:http://mark.zoomcities.com/images/gfx/GFXpowerchartby3d.png
You'll need to zoom in, but you'll find that the cards you're considering appear in roughly the 80-110W peak range, with the 9800GTX+ (65nm models) requiring the most of them all.