Your PSU barely has 28A combined on the +12V rail, which is very little for a new card. I'd recommend getting a new PSU like the Corsair 650TX, which can be had for around $80 AR on Newegg. Oh and FYI, the GTX 280 requires the same amount of current on the +12V rail as the 9800GX2, where did you get the 26A number from?
As for a new card, go for an HD 4870 or the GTX 280.