For the applications you intend to use, the length of time and the cost differential its academic - I really don't think you are going to notice the speed between the two until you end up upgrading to CS5 etc. within the next 18 months.
I'd also forget about the 'might' with RAM speeds and 16GB for now is going to cost you a small fortune. By the time the 4GB memory prices go down your apps will be faster on 8 core and you will have already upgraded to 16GBs had you gone with the 8 core.
I'd also forget about the 'might' with RAM speeds and 16GB for now is going to cost you a small fortune. By the time the 4GB memory prices go down your apps will be faster on 8 core and you will have already upgraded to 16GBs had you gone with the 8 core.