Bah. That's not good. An 8600GT is suppose to have 32 shaders (clocked rather high I might add) though I see you got a cheap DDR2 version. Anyway, it could be the card is dying which is why it's not doing as well. Pretty odd that it's reporting 16 shaders as that's what the 8500s had, not the 8600s.
The 550Ti is faster than the 6770, but uses more power. Also, a GTX 460 is a much better buy than the 550Ti but of course that's a bit more power hungry ^_^. If you have a good PSU get a GTX 460 or even consider a Radeon 6850.
No, don't get that PSU. It's a no name one, so unless you do your research and can be sure that it's actually a re-branded high quality unit you risk breaking your system by using a poor quality power supply. Anyway I'd say at the least you should invest in a decent PSU like
That should be more than enough for just about any single GPU. Again I'd say go for the GTX 460 or a Radeon 6850 (the 6850 is a little faster and uses less power) rather than the 550ti which is closer in performance to the 6770.