Thanks a lot. I've done some more research and it appears that even though a single 6P can be converted into an 8P, it officially supplies only up to 75W of power while the true 8P connector provides 150W. There are varying opinions on whether such a converted 8P connector should or should not be used (as the 6P usually can pump more than 75W), and the manufacturers recommend against using such converters, at least for the 9800GX2 cards. I'll try to locate a connector and attempt running the card with my old PSU first, failing that I'll need to get a new PSU (provided I haven't killed the 280 already).
What would be a good silent PSU with long cables in the 600-800W range?
UPDATE: apparently the converted 6to8P connector can only reliable pump up to 108W and since the GTX 280 draws as much as 300W under load I've picked up a new PSU instead of doing the converter thing. My local store had the Corsair TX750W for $125 and it has 4 6+2 connectors so in theory it will handle the 280 (or two of them if I add one later).
Thanks for all the helpful suggestions guys. I just hope that me stupidly jamming the EPS line into the PCI connector on the videocard did not fry the 280.