Intel's i3s can beat many Core 2 Quad CPUs and at least match the top models, if not beat them too. Most of the LGA 1155 Pentiums and Celerons are still great yet cheap upgrades coming from a Core 2 Duo, granted with Haswell seemingly right around the corner, it might be worthwhile to wait and see if the situation improves even more with Haswell.
i3 did not beat the top core 2 quad yet in multi-threading, and i3 price isnt really cheap as well, it cost as much as a used core 2 quad, add in a RAM change + mobo change, that is nowhere near cheaper than changing core 2 quad alone.
On Ebay, the cheapest Core 2 Quad/Extreme that I could find that even has a chance of not being beaten by a decent i3 in even highly threaded performance was a QX6850 for $160. That's cheap enough for an i3 paired with a new 4GB DDR3 memory kit and a cheap motherboard to almost fit in and the power savings can easily cover the rest given a few months to a year (granted probably not much else).
Going for a top LGA 775 Core 2 Quad to keep i3s at bay in highly threaded performance would mean going up to around $250 for a QX9650 and even then, the top LGA 775 Core 2 Quad/Extreme isn't found until over $300. So, i3 is clearly winning on the price front, at least on Ebay. IDK about other sites.
The huge lightly threaded performance improvement is also noteworthy.