I use a Quad and love it. I render things about twice as fast as my OC'd 6700: very helpful for heavy renders (I use Cinema and Maya). 

Here's a chart, (from Tom's Hardware), comparing different multithreaded render times...Intel Quad pretty much leaves every other processor in the dust. 

http://www23.tomshardware.com/cpu.html?modelx=33&model1=604&model2=465&chart=188

If you have the means, I highly recommend it :)