I was thinking about upgrading my 8800GT to a GTX550ti, but im not so sure if I would notice that drastic of a performance increase.
I normally play at 1920x1080 and in a few games like the Witcher 2 and Hydrophobia my card is starting to show its age.
Shame on me for using passmark, but I still think it would be closer to 60% faster, plus the original question was only asking if the increase would be drastically noticeable, which I believe it would be. Plus it's still a respectable amount of power for $120!
I upgraded from a 9800GT (same as an 8800GT) to a GTX460 (that I overclocked a bit using software included by EVGA). I noticed frame rate improvement of 2 to 4 times depending on the settings....Basically previosly with a playable framerate (based on settings I would get double frames with the 1GB GTX 460. However, the 460 was about 3 to 4 times faster if I turned on more AA and the such. So lets say I would go from 15 FPS on the 9800GT I would get like 50 to 55 on the GTX.
I think Tom's benchmark indexes help support this as they are FPS compilations of games
I believe the 880GT recommended a 400W and the 460 recommends 450W. so it matters how much you trust your PSU. I am running a questionable quality 550W power supply that is 5 years old and it works, but I am not an expert on PSUs...
the GTX 460 needs 2 6 pin connectors but I think it comes with adapters if you only have 1.