As someone that built a new personal computer about every 9 months for many years, have to agree with Kash. During the 90's and early part of this decade, things were doubling in speed every 12 months at a minimum. Back then, if there was word on a new processor or video card coming out in the next couple of months, they were typically worth waiting for, only because you could expect a doubling in speed compared to the one you bought last year.
Upgrading today really offers diminishing returns from what was happening in the industry during that time. There will always be something newer and better. But, for the last 4-5 yrs, the only thing of real note has been the change from the P4's to the C2D's.
My now 2 year old MBP as an example - a 2.33 Ghz with 256 MB video. The latest and greatest MBP's only at 2.4 & 256 or the 2.53 with 512. Still have 2GB RAM in mine and have only replaced the hard drive with a 7200 RPM drive. A friend that has the 2.4 with memory upgraded to 4GB, we ran an Xbench test. My 2 year old machine beat his Base score by 10 points. His processor beat up on mine a little and my video card and hard drive beat up his.
This really points to the fact that today, the time to buy a computer, is when you need it. If you don't need it, then wait. If you want it, got to have it now, not a big deal, and you'll not be left that far behind and you get to start enjoying that new toy now. Don't worry, be happy.