The newest Intel processors with hyper threading tend to do better with video encoding, however
AMD processors are much more economical. I recommend this site
http://www.tomshardware.com for more details. However, in my experience, especially with inventor, ram is more critical then processor speed. I would personally use an AMD processor and use the extra money to invest in more ram. 512 should be a minimum- shoot for at least a gig, preferably two.