Are you trying 3d with the ray traced renderer? How much do you use that feature? If you don't use it, a Quadro is not going to help you. What is your current graphics card?
What other tasks are you trying to accomplish? Do you have a solid state hard drive for your cache? Have you thought about adding more RAM?
No I dont't do 3D, just trying to play with some effects and even the previsualisation takes quite a bit. I'm on Win7x64, 12GB ram and my current graphic card is a ATI Radeon HD7800 with 2GB ram.
From my understanding the ram previsualization doesn't use the hard drive, right? I've got 2 HD @7200 rpm.
about GTX card - haven't thought about them, I thought they were more for gaming.
...even the previsualisation takes quite a bit.
Please define what you mean by the term, "quite a bit". Your present experience with AE may be perfectly normal, but we can't tell.
"Quite a bit" can mean "not instantaneous" in some peoples' minds. For other people, it could mean "a day and a half".
Creating a RAM preview is just like rendering and so it does use the cache to speed things along when possible. I mean, you've got footage in your scenes, right? It probably won't help much, but it might help a little.
As far as the benefit of a new graphics card, you really need to look at the link Todd posted (and the links it contains). If you're using CS6 (which is actually AE version 11, not AE 6 as your post says), then you need to pay special attention to this link. A new graphics card upgrade will only help you a teeny weeny bit, if at all. And you should definitely not get a Quadro in any case; for what you're doing, that's a waste of money. The cards listed here will all do. I'd get a decent GTX (unless you're running some other software that would benefit from a Quadro).