Hi there my 8600gt died and am thinking of getting either a GTS 250 or 9800gt as replacement, cant afford any better. My PSU is a thermaltake PurePower 500W with 14 and 15 and on the 12v1 and 12v2 rails respectively. The GTS supposedly needs combined 30 amps on the 12v but im not exactly sure what that means, because it will only be using one connector. Any help will be appreciated!
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.