I hope someone will provide more detailed advice but I'll certainly help you think this through. You haven't said much about the type of encoding you do, the degree of compression you require, what's being re-coded if anything. So my offered advice is generic. I've also assumed that your video encoding software is nothing high end.
1/
Video encoding software:b I recommend that you research for software that actually can use multiple CPUs and ideally can utilise the processing power on a top line Nvidia graphics card (but if budget is limited forget the graphics card suggestion). If you find such encoding software, then Quad CPU is twice as good as a single Dual Core.
2/
For video encoding, CPU power is everything. The INTEL web site will tell you which CPUs have 6MB level 2 cache. So this will be a budget driven decision.
3/
A 64 bit OS matched to 64-bit encoding application will work faster than its 32 bit counterpart because of the data bandwidth. So again, it comes down to researching the encoding software you want to use. If you go 64 bit OS, take 8GB RAM at least so that your software can buffer optimally.
4/
If you're using freeware to do the encoding, then it's processor power only that matters.
5/
50C heat doesn't matter. Don't worry about it.
In conclusion, it's the software you intend to use that's key and if you stick with what (software) you've got, don't expect serious improvement.
