ur PSU will handle 9600GT fine. i run my 9600GT with amd 5600+ with a 420W PSU.
yes the performance gain with 9600GT over 4670 will have its mark felt specially if u game on max resolution with max setting. I get 35+ fps on all games with max settings @ 1440x900 (19'in WS). so going into 4670 might result in lower than 30 fps which is ok for some, but i like 35+......
but if u go into 22inch, den definitely u need 9600GT.actually a 8800GT will be better
as for bottleneck, i feel that on certain titles (IE: crysis warhead, assasins creed, grid, etc.) my CPU @ 2.8GHz bottlenecks ma 9600GT. however i still do get fps of 35+. so i guess it should be ok