Umm.... CPU at 100% is bad and will cause an immediate lag. In BF3 multiplayer your CPU is holding you back and hence why the i3 is not what everyone claims it to be. If an i5 3570k is hitting 60% with 4 cores then there is no hope for an i3 with 2 cores.
4 Gigs of RAM is very borderline, and if your system needs to use the Hard drive if it runs out of ram, that will be painfully slow as well. I can comment on both these issues from experience. The best thing you can do is research CPU usage and RAM usage for the games you play.