For starters do a clean installation of the latest Nvidia drivers.
Then have a look at background applications, do you have a load of stuff running at the same time?
Then check your temperatures while ingame, if it's getting very high it could be throttling.
your gpu is kinda old, try an AMD 7850/70 or if your budgets goes 7950/70
Seriously? I was playing BF3 on a 460 on medium settings 1080p without any problems, so that's complete rubbish.