If we're talking individual components, I upgrade as I see fit which could make 12 upgrades a year, though for this I considered an upgrade being a mobo/ cpu upgrade.
I try to buy around 2nd or 3rd best cpu, and best mobo. The reason is because the price on cpu's have stabilized to a point that you can upgrade down the road, and sell the old cpu for very close to what you paid for it. IE; Buy a 1700xp now for $120, wait til the 2100xp gets down to $120, sell the 1700 for ~$100. I tend to upgrade mobos when the cpu I want to upgrade to has a feature not supported by the current mobo.