I just wanted to share with the rest of you how successful I have been with running Dynamips on my PC.
My specs:
Intel Quad Core Q6600 - overclocked to 3.4 GHz (default clock is 2.4 GHz)
4GB RAM
Ubuntu Linux v 7.10 - 64 bit
I'm currently doing my Vol II SP labs which consists of about 12 router instances (7200s and 3600s). I've done both lab scenarios 1 and 2 so far. At the end of each lab, my processor cores are only running between 20-40% utilization. With 4GB RAM, I'm able to run everything in a single Dynamips instance.
Even without overclocking the processor, you should be fine with the above setup. But, the Q6600s are great overclockers - if you are interested in that.