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The tricore will be significantly better at encoding, though still rather behind intel I'm afraid.
Multitasking needs to be defined really. Are you finding that the computer gets steadily slower as you ask it to do more things at once past a certain point, or are you deliberately loading entire cores?
If it's the former, then check your ram. If you're running low, that's the point to upgrade. If not, it's hard drive or processor that's holding you back. % processor used when running slowly is good for this, on xp hit ctrl-alt-delete then minimise it, you get a little box in the bottom right corner which goes all green if the processor is fully loaded i.e. limiting you.
The F1 hard drives are about as quick as any you can get, so you can be fairly sure it's the processor causing issues. Whether it's raw clock speed or number of cores you need depends on use. For gentle multitasking, say office + firefox + antivirus scan + media player, a faster dual core is the answer. Once you manage to put significant workload on it, multicore pulls ahead. For example, if I wish to run debian, ubuntu and xp simultaneously, a dual core is having none of it but a quad is just fine.
I suspect its raw clockspeed you'll feel the most, which suggests overclocking rather than an extra core