Disclaimer: Answers based on reviews and tests and views on the direction where the SW&HW industry is going...
Gaming: According to test results which can be found for example from tom's hw articles i3 is the better CPU for gaming.
i3 2100 and i3 540 are close to the same, as you can OC the 540 to the same clocks as the i3 2100 or basically about as much as you can the 955BE. And at the moment they perform better at gaming than the 955BE. So in this case I would have to say i3 540 overclocked. i3 2100 in stock.
Overclocking:
This is a tough one: i3 540 and 955BE are probably very close. But 955BE is in a sense better because it has an adjustable multiplier, while i3 540 OC is done with FSB. i3 2100 is not an option really since it can't be OC'd any meaningful amount.
Future proofing: More of a platform/socket thing really.
This pretty much rules out the i3 540 completely, due to the dead 1156 socket.
i3 2100 has a future when paired with the proper P67 or Z68 motherboard. Allows for replacement CPU's to the 1155 socket.
955BE works in both AM3 and AM3+ MB's so with the proper AM3+ motherboard this is also future proofed.
But thinking strictly of the CPU 955BE is the best because it has 4 cores , and games and other SW is constantly evolving to utilize more cores.
So the answer to this is: CPU wise 955 BE, platform wise it's pretty much a tie between the 955BE and i3 2100 possibly leaning a bit towards i3 2100