As far as I can tell the 550ti is only inferior in the memory pipeline area however in performance it is acting like it doesn't exist at all.
To elaborate, I'm seeing FPS of around 5-9 in games that the 8800gtx ran at 60fps (Eve Online).
I uninstalled the old drivers from the device manager before installing the card, but this caused windows to stop booting up outside of safe mode, I did a system restore & overwrote the old drivers with the newest(301.xx?) and for a brief moment the card worked seemingly as it was supposed to(4 boxed EO no problem, tabbed from window to window instantly) but after rebooting....it went back to performing like the computer didn't have a motherboard, I have since updated the drivers to 306.xx which came out just a few days ago, but the performance remains unchanged.
GPU-z seems to detect that the card is there
MSI-Afterburner/Kombuster was having the card report 35-45 FPS at 99% load I don't remember the numbers but it was like 2200 objects on screen or something?
It sounds like you are having driver problems. Try downloading a program called Driversweeper, and remove all your nvidia drivers with it. Even using the nvidia uninstaller you can sometimes have residual stuff left behind that can cause problems. Then install the latest driver for your new card.
Also, what temps are MSI Afterburner reporting on your card while under load? If the temps are getting too high, then the card will throttle performance to protect itself from damage.