It will, however, use a lot less power than the Q9550, plus put out less heat. A tradeoff.
A 2120K, if/when/how much it shows up at, should take care of any performance concerns while keeping the low power/heat.
		
		
	 
OK, if he's going from a e5200 to a q9550 or i3 2100 why would he have any performance concerns between the 2 choices?
I say get what you can afford now.  Sitting around playing the waiting game is pointless.  There is always something just around the corner.  If you waited, you'd never upgrade because you can wait just a little longer and get something ZOMG better.
I would go with the i3 setup and be done with it.  It'll run cooler, use less power and give close performance with the q9550.  And the board will let you upgrade to the higher end Sandy Bridges when people start dumping them to get into the 2011 setups.