I don't advise that anybody try to keep up....and I don't advise buying top of the line either.
My recommendations are to buy a generation or two back....and get something reasonably affordable. Then use it for 2-3 years, then replace it. The money you save on not buying the top end hardware will end up paying for the new computer.
I bought my current box in July of 2009, it's a Q9550 quad core (at the time, the original Core i7 was top dog). Now, over 2 years later, it's still quite a powerful machine. The only thing I have changed is my video card. When I bought the box in 2009, I bought an oldish nvidia 9800GTX+ which i always intended to replace. But I didn't actually have a need for the past 2 years. With BF3 coming out, I just bought an Nvidia GTX 570. I was able to play the Beta on high settings no problem. My CPU didn't break a sweat. I don't see any reason to believe this machine won't continue to be fast for at least 1 more year.
Sure, there are faster machines out there, but I don't really do anything which requires more speed. I can already encode video fast, I can rip DVD's in just under 8 minutes, I can run virtual machines, I can play the latest games, it just works for me. Of course, as a gamer, I play on a single monitor with 1920x1080....so I'm not the be-all end-all poster child either. Your needs and expectations will differ from mine. Staying on a machine might not work for everybody...but it can probably work for many.