IMO, the smallest step up from 3GHz Core2Duo worth considering at this point in time would be i3-3220 but that would mean new MoBo+CPU+RAM worth ~$270 total. For ~$30 more, you can kick it up another notch with i5-3350P.
Considering how ridiculously expensive DDR2 is today (3-4X the cost of DDR3), the platform upgrade enables significant savings there, since you will likely want to upgrade to 4-8GB RAM.