ill just endure that ''old'' 5750 and get a gtx460 later on
When installing a new card, I simply run driver sweeper, and install the new Nvidia drivers right?
will it delete catalyst and everything related?
You really have no choice as far as a gpu goes since your mobo isn't true xfire.
A GTX 460 is an excellent choice in either form 768mb or 1gb.
Unistall ATI Manager in ADD/REMOVE Programs first.
Then run Driversweeper in safemode to purge the remaining ATI files.
Install new Nvidia gpu and drivers after.