You will see a little bit of improvement if you are playing heavy CPU-bound games such as Dirt3 or BF3 Multiplayer going from an i3-->i5. You will see little to no improvement in games going from an i5-->i7 for gaming
BF3 on large multiplayer maps would benefit alot from the i5 other than that unless you are playing at a very low resolution there is little benefit except maybe a few CPU intensive games (none I know about).
You see some improvement, but not much unless there is a specific game you want to play that can take advantage of 4 cores.
At this point in time I would say to save up for a quad core Haswell that's coming out next year and motherboard for it. How much will a Haswell cost? Unknown, maybe around $240 - $250 for the unlocked quad core version.