Quad cores are awesome if you are doing the above: Ripping multiple jobs simultaneously. Multiple meaning 3 or 4 really. The issues is that as far as I know, none of the rips can multi-thread a single rip job. IE, the benefit of a quad core is really lost if you tend to rip one job at a time. In that case save yourself some money and go with a core2duo. In fact, in a 1 job at a time scenario the clock speed will be more important than the number of cores.
So to summarize, ripping a lot of jobs at once, go for the quad. 1 job at a time, go for the core 2 duo.
Finally, I really recommend having separate RIP and design PCs. My office is next to the print room, so I have our RIP computer and a design workstation in there. I found a neat piece of software called MaxiVista, which allows you to control two separate PCs via 1 set of controls really seamlessly. It gives you a ton of configuration options so with a click of a button I can go from a dual head display of my design computer to a side by side view of my rip and design stations. Again, both computers controlled by 1 keyboard and mouse seamlessly.