Before I used to upgrade like nobody's business, but starting like '06 I've made it a point to keep a 2-year upgrade cycle, but to specific, here's the breakdown by parts:
CPU/Mobo/RAM: 2yrs
VC: 18mos-2yrs
Casing/PSU: 2yrs+
HDD/ODD: Whenever me good deal sa price (e.g. P3300 for a 400gb sata, like recently

)
Soundcard/Misc stuff: same with hdd/odd
Speakers: I've been rocking my Z680s for 4-5yrs na , so no plans of upgrading. I did complemented it with my Audio-Technica ATH-ES7 cans 2-3months ago.
So that's my general guidelines, konting control lang sa gigil, alam naman nating medyo mahirap ang pera these days,
BUT if you have the cash, and know what you're buying, go ahead and splurge
