If you want fastest encoding possible, almost nothing beats a more powerful CPU. Your best bet is an Core i7, then i5, then i3.
Pick the most important thing to you and buy your CPU based on that. You can play HD movies with your Core2 Duo Intel or any number of other chips.
My point is, quit worrying about i3 vs i5. If encode speed is what you need, don't stop at the i5. A current gen i7 will do my encoding in about half the time that it takes my current AMD Phenom II 955BE X4 (which is a good bit faster than your current cpu). If you can't return or exchange your i3, sell it - and buy an i7.