The 285 (Tonga die) is definitely not a crippled 290 (Hawaii die), although it is essentially a replacement for 280 which brings in some Hawaii features like the ability to draw 4 triangles per clock plus some new features we haven't shipped in any GPU/card so far.
There are three generations of what we call GCN (Graphics Core Next) - Southern Islands (SI), Sea Islands (CI) and Volcanic Islands (VI). The 270 and 280 are tweaked SI (fab process & power management improvements), 260 and 290 are CI along with Kaveri, Kabini and Beema/Mullins APUs, and 285 is VI.
During the Hawaii launch the 290 was described as VI but that's "marketing VI" not "engineering VI"
VI adds a new instruction set for the shader core (the 1792 or 2048-wide processor core) and improved colour compression - it's colour compression that allows a 2GB / 256 bit memory bus card to perform like a 3GB / 384 bit memory bus 280 or faster.
I'm leaning more and more towards the 285 for you unless you think your next purchase is likely to be a really high res display, greater than 1920x1080, or running a multi-screen gaming system, in which case the 280 or 280X might end up being worth the extra $$ because the 3GB RAM will help a bit at higher resolutions.
I do think the 280X should be OK with your 650W power supply but I don't have any direct experience since I just happen to stick 750W supplies in all my systems and all our boxes at work tend to be configured the same way. Not only CPU makes a difference but hard disks -- I suspect the 700W rating assumes a couple of hard disks, a floppy, an optical or two etc... but again I'm not 100% sure.
