when to upgrade pc?

the BEST time to upgrade is JUST when new components comes out 8)
BUT that's also when the components are the most expensive.
you need to trade off when to upgrade and the cost of upgrade so there is no real answer to when to upgrade.
if you have unlimited budget, then sure, upgrade every 3-6months to the latest and greatest.
most normal people upgrade anywhere from 1yr to 3-4yrs and most of them will just get new pcs.
either way, just remember that the price of any component on a pc usually drops by half within a year, wont even be sold anymore or pretty worthless.
it's just how computer technology is and progress is always good.

Over the years I have built 6 PC's. I find that you get the most bang for your buck (price/value vs. performance) if you buy right below top of the line. I am a gamer, and the only exception I make here is a video card.

Upgrade with each DirectX major change.

Actually, no, I upgrade every 3 years or so. But, the way the years have fallen, it basically worked out as DX upgrade anyway. (Geforce 1, then Radeon 9800, and I'm scheduled for a new rig at the end of this year/early 2007.)

I would also say perhaps look at stuff other than the obvious chip/socket changes, which happen pretty often anyway. I look more towards longer term changes (hopefully), such as PCI-E, SATA/SAS hard drives. DDR2/3 memory etc.

Ronaldo38741

There isn't an answer because it is a subjective question. As for a PC's real lifespan, I typically keep a machine around 3-5 years before I get another one. It's not because I don't have the money, it's because I really can't justify the expense when my current computer does everything I need. But that's the answer for me. For you it may be different.

After 3-5 years, I give the machines to family. Usually, it gets another couple of good years.