I can tell you now as someone that actually understands and knows this shit......go the Quad core. Although Protools doesn't Multithread (In English......written to be used by and fully take advantage of a multiple core processor by assigning different processes to a different core) you will see a greater response from the quad core than the dual core. A Quad-Core will always preform better than a Dual-core with Hyperthreading...always.
Also the larger cache will greatly reduce wait times for CPU instructions, which will increase the overall speed of your PC (and Protools). Also, yes extra RAM will help, but if you've got a 32-bit Windows operating system, you can only use 3.5gb anyway as that is all that 32bit Windos OS can address. 64 bit Windows OS's can address more RAM, but they are still "unsupported" by Protools...though plenty of people have got it working fine.
End of the day, Quad core and 64bit OS are the future, its not long till AMD release their 6-core processors anyway and this argument will be null and void. But if you are going to be using this for Protools and Adobe Premier....Quad-Core and 32 bit OS is the way to go...not only will they run great, your general PC usage will be relativly quicker than a Dual-Core PC...though like I said, 64bit OS is fine if you can get Pro Tools to work on it, which many people have (plus its not long until Digidesign wake up to themselves and release a new version of PT that supports 64 bit).