If your on a tight budget the I3 is a good choice, it handles most games really well, and is a huge upgrade over your dated X2 6000. Depending on what your video card is and what resolution your gaming at there may be no benefit of moving to an I5.
I've got the X2 6000 and I3 2100 compared here.
http://www.anandtech.com/bench/Product/28?vs=289
If you can get the 2500k for 150$ and 60$ off a Z68 mobo, do that!!!
How would one do that?