Will a 2nd 6600GT boost your graphics performance?
Yes it will.......to around or slightly above a 6800Ultras performance.
You will be able to add atialiasing and AF at reasonable resolutions in all games with that.
On the other hand a 7800GT......actually you need the much improved and faster (and cheaper) 7900GT is significantly more powerful than 2x6600GTs....probably nearer 4x6600GTs (or 2x6800 Ultra)
It is worth updating your 6600GT by doubling up....definitely.But you wont be getting anywhere near the performance of the latest £200 plus cards.
If you are having serious gameplay problems with any games on your single card I would advise saving a bit more for a 7900GT but if gaming is not your absolute priority or if £200+ is too much you wont regret adding a second 6600GT.