Fairuse supports dual core CPUs, I am not sure what kind of perfomance increase you'll get in quad core as opposed to dual core.

http://www.fairusewizard.com/lang_en/fairuse_wizard_dvd_divx_xvid_backup_tool_buynow.html

I tried this on a quad-core box recently.  It doesn't appear to be threaded for more then 2 cores.  Given the nature of encoding though, I'm not surprised that you can't get much more benefit from additional threading.

I believe that thread support is more of an issue with the codec used rather than FairUse Wizard.  I have been using multi-core processors with FU and the x264 codec. Initially I ran on a dual-core and the speed-up was somewhat unimpressive. I was using about 60% of the processor capacity which is only slightly more than for a single core. Then I discovered a place to specify number of threads for the x264 codec. If you go to the 'Misc' tab of the codec settings you'll see a box for entering number of threads. I reset this from one to two threads and got about 75% CPU utilization. Not fantastic, but better. Setting threads to 4 managed to get a quite respectable 90-95% utilization which meant I was getting very close to double the speed of an equivalently clocked single-core CPU. When I got a quad-core, the best I could do was 75% utilization. This was because the x264 codec supplied by FU 2.6 would only accept values of 1, 2, or 4 for processor threads. I'm happy to report that this has changed in the FU 2.8 release. Setting threads to 0 (auto select) now allows me to get 95% CPU utilization. So, with the proper version and proper settings, you can get pretty close to the expected speed-up from a quad-core CPU.