Different types of memory are used for different tasks, RAM (or VRAM) is used because it is very fast. What more VRAM allows you to do is use more software at one time, multitasking has nothing to do with memory. Actually the only way you can truly multitask is if you have parallel processor's (think Graphics Processor's) but you won't really be using it directly anyway because us humans will never be able to multitask under normal circumstances.
A couple of notes, the amount of cores on your processor does not matter as much as the frequency of hertz, and the frequency of hertz does not matter as much as integrated processor memory. The larger the bus bit-width is for your GPU the more data your GPU can transfer at one time, 256-bit > 128-bit.
The best processor's are the one's that use power per core efficiently and have a large cache.
Also it depends on what type of applications you are going to use and if those applications will use more cores. It's also important to take into the account of threads versus cores and how they differentiate. Threads are virtual representations of a programs instructions to be executed by your logical core's, the more logical core's you have the more thread's you can execute at once. If you have SMT/HT each logical core shares some of it's processing power between two thread's at once so you effectively get one logical processor processing two threads back and forth, thus creating the illusion of two more logical processors. Which is why if you have a processor with Hyper Threading technology, proprietary Intel, then you have four virtual cores if you have two logical cores.
Anyway, just worry about this when you actually have the money.