This is an age old question that is still often asked. The only times I ever recommend to buy off the shelf desktops or laptops is either:
- Black Friday specials if you're willing to get up that early.
- Announced price cuts from either AMD or Intel
- New generation of video cards or chipsets or whatever is really cool that is coming out in a 1 to 2 months time frame.
Right now, CPU prices (thanks to Intel's very aggressive stance vs AMD) are pretty ceap. RAM is really cheap, I paid $220 for 2gb of DDR2 in Jan '07, you can get the same stuff sub $80 now.
As for new technology to wait for... I haven't followed super close to much. The Nvidia 8800 (not 8500-8600) are solid cards, and ATI is finally coming around with their newly released cards. I heard something about DirectX 10.1 coming up, but I haven't looked into it. New processor generations are $ when they first come out.
IMHO, it boils down to is your new computer dreams now become a need or a want. Does your budget allow you currently spend on a computer. Because when you play the waiting game (lets say for 45nm), there may be a new cool chipset or video card generation just a month or two away. And you'll wait. And your current computer will still suck. Hopefully, you'll then find that the majority of the time, the best time is now. Otherwise you'll always be waiting.
When I build a new computer, I may save up money for that new generation of cpu or whatever and play the waiting game. And when I did wait I saved more money to buy an even nicer computer than if I bought now. But eventually you'll have to byte

the bullet and buy something.
So ask yourself: What is my budget? (think ahead for money saved down the road if you wait) What are my needs? (quad core whatever)