Whenever a new series of products is released, or there are a lot of price drops, I upgrade.
For instance: this fall, the Asus A7N266 (nForce chipset) motherboard will be out, the Athlon 4 processors will be up to 1.5 GHz+, Crucial PC2100 Cas2 DDR SDRAM will be out (right now it's Cas 2.5), and the GeForce3 will have lower prices (and there may be a GF3 Ultra out too). That's when I'm planning to upgrade.