What properties to look for when buying a computer?

What are the important properties (i.e., processor, processor speed, memory, etc.) and how important are they? I'm not a gamer and I don't watch movies on my computer. I tend to run a lot of programs simultaneously, and they seem to be memory hogs - my current computer often seems quite sluggish (yes, my anti-virus, etc., software is up to date). I'm learning to use Photoshop and also learning to program in Flash, and I'd like to start learning to use Blender. I can't afford to get a computer with the best of everything, so what should I look for? The most memory? Fastest processor speed? Something else?

I'm most likely going to get a laptop, but I am considering an all-in-one.

Thanks!