Ok I just saw the one of your other 6 freaking threads where you actually posted the PSU info. The PSU should be fine, and if you had a CPU bottleneck it would be close to 100% load, I don't think that's the issue either, not with a NFS game.
as already has been said, the game your running can be run on a much lesser system than you already have prefectly fine, not only that, but it takes no advantage of 4 cpu cores, let alone 2. So getting a quad core isnt going to help. Things you should do - defragment your hard drive, reinstall the game, make sure your hard drive isnt too full, this will slow things down, check all the fans are running, CPU fan, GPU fan, PSU fan etc and check cpu and gpu temps under load, not at idle. Run Memtest86+, run your hard drive manufacturers diagnostic tool, and you may possibly need to re-install windows. then get back to us.