Photoshop and Illustrator perform better and longer on a more powerful hardware with plenty of RAM.
Also since using those programs also requires doing a lot of other things on the machine at the same time (like listening to iTunes or surfing the web for images) and switching between them (like doing something else while rendering) the more processor cores and RAM the better.
Apple is mainly a consumer products company, the newer dual core machines are catering to that markets wants for just a basic surfing, watching video machine.
Professional users need a bit more punch and since the quad core with it's stronger graphics will last longer with the high costs of that sort of software, the less hardware turnover the better.
The dual core is underpowered for your intended use and will require a new machine in a shorter time, which in turn will require newer versions of upgraded software which is more money.
If you really want to save money, then get a MacPro when it comes out and then a light dual core laptop for general/Internet use and keep the MacPro offline except for updates and verification.
That way you can use the MacPro for 7-10 years easy and not even have to upgrade software if you don't wish.