Personally, I you do have to scope the market before you buy a computer if you're putting a great deal of money into it and want it to last for 2 years (my personal time span for a Main Computer) and keep doing exactly what you want.
At Christmas I thought, when I brought my computer, was a good time. You have to think first; how long do you want it to last, what is it you want to do with it, and how much money you have to spend on it.
DDR2 is good, as is PCI Express - point is would you buy these when they come out if you wait. Would you want DDR2; would it be in your budget. Would you want PCI Express; or would you want to keep with the AGP which I personally believe will beat the PCI Express in time simply because it is a better design.
If you wait too long, the less you have your computer before it depreciates. Another thing is do you need a new computer, or can you wait for a market flat to come up in order to buy your PC like what happened in Christmas and, even a surprise to me, is still basically happening now (7 months only Intel+400Mhz/AMD+400 which is amazing).
There are a lot of things to consider. I'm happy if you can answer those questions in another post to help you think about it; but really its something you should do. Buy PC Buyer and flick through the Adverts. Look for a good offer, a money off, a buy this and get this free. Then think about how much you have and how much you could stretch that, then if you need a new one right now.
But personally, even with DDR2 and PCI Express - this is a good time to buy a PC - but not one of the best times.