For gaming on Linux, the way to go is Wine and/or Cedega, like said above. Cedega costs (there's a version you don't have to pay for, but it doesn't come with "support" and I never got it working well personally), but on the other hand it ought to play your games better than plain Wine. However Wine plays some games too, even though it isn't directx-maniac.
The single/dual/quad/n-core question is more than just specs on paper; refer the other threads here for information on how separate single and multiple processors and "n-core" processors differ from each other in terms of performance and abilities. Not sure if there is one correct answer, but one thing is for sure, and that's that you can't just add up the MHz values when comparing processors.
And if you really are going to save (how do you do that, by the way -- living like dead in terms of food and warmth, maybe?
just joking..) the bucks to be able to buy a PC you described, especially with a high-end nVidia graphics card (or any other brand for that matter), then consider this: very probably you'll get a decent computer for your daily needs and
a shiny gaming console (PS3, XBox360, you name it) and
a few games with the same amount of money. That means a machine that is designed for gaming (means smooth gaming, no driver or OS woes) and a machine that you can do your work on. So..what's the point in buying a crazy-sized PC for gaming if you can get two or three equally good gaming consoles for the same money, or a gaming console and a hdmi telly?
I just wonder..because a "good", as in gaming-good, (in your case) nVidia card costs more
than PS3, which is the most expensive gaming console on the market. Add the PC specs you said and you can buy a television too, large enough not to fit in trough your door.
Think about it.