never had any real fps issues with gta 4 and my system doesnt have a 680.
most of the time bad performance in this game is down to other crap on the pc.
open taskmanager and see howmany processes are running. on windows 7 it should be between 52 and 60 processes. any more than that and your system is being bogged down by un-needed processes.
seriously i can get gta 4 running at max settings for a 1 gig card on a 8800gt amd 6000x2. all because the system is well optimized in the o.s dept. it plays slightly better on my sig system giving a solid 60 fps where as it was 30 minimum on the 88gt.
the odd problem from the game did creep in where the textures would disapear but patching soleved that... 1.4 is probably the most effective as the higher versions are pretty much dedicated to the online portion of the game...
to optimize your system use something like windows 7 manager and run its optimization wizard just go step by step answering the questions. do it properly and you will substantially reduce the backround processes and free up cpu cycles which is what the game needs.