whether your building a computer or buying one i recommend getting one that has an intel quadcore Q9550 or Q9650.  get a motherboard the has a front side bus of 1333 to 1600.  at least 4GB of DDR2 ram, best would be to get 8GBs of ram (try getting DDR3 ram). now nvidia has great graphics cards dont get me wbrotherrong but i have seen the "Radeon ATI 4870 HD X2" and all i can tell you is WOW! visuals are great crisp and clean. great for gaming. now on the adobe after effects side it works great (being that you have the parts described above). i dont know about after effects CS3 but after effects CS4 has this option where you can choose how much ram you want to use or CPU (meaning processors) to render and preview your video project. there is also an option where you can tap into your graphics card and use the ram that your graphics card has.  it improves the playback greatly so you can view your effects that you have added on to your video without any horrible lag and chopp movements.  hope this helps! ohhh and another thing you should try a fry's electronics.  they can build you a computer.  hope this helps. later!