Online Gaming separate from Turnkey System

Sorry for an off topic question I guess.

I'll be purchasing a turnkey system either in the Apple or Windows world soon. I also do a lot of online gaming. And I plan to buy a separate system for that, and other internet browsing, desktop work , etc. alone.

Somebody already said to just buy a suped up Mac Pro and put my games on it. But I don't want to cross the streams.

What is the cheapest option I could buy for a stand alone internet/gaming system and still be able to play the games fine?

I was looking at some computer in the box systems at Best Buy the other day and they seemed to want to charge the price of an i3 or i5 computer for a lesser processor, and I'm too much of a tight wod to do that. If I'm gonna buy something at that price it better be for an i3, i5, or i7. Of course the i7s were very easy to spot because of their price tag.

In order to play a current online game today, like Black Ops, or some MMO what can I get away with and still have a computer that can perform well.

I'm not IT savi. I know what is labeled as powerful sort of when it comes to the above processors mentioned. But I have no idea about lower processors, or esepcially AMDs.

Thanks for any insight