I used to do the exact same thing....but now that free online classifieds (Craigslist etc) exist, I'm starting to think its smarter to buy a whole new rig every single major GPU release and then resell your old one for 75% of what you bought it for...
Dodgy math inc:
Scenario 1. Spend $2000 on new rig, expect 4 year life. At the end of the 4 years you might be able to sell it for $300 if you find the right nerd/sucker. That's $425/year expense on hardware assuming you don't do any upgrades at all.
Scenario 2. Spend $2000 on a new rig right at launch of new GPU generation. Use it until launch of next gen (usually 12 months or less) buy a brand new rig, and sell the old one for 75% of what you paid for it. That translates into $500/ year hardware expense and you always have a brand new PC... Thinking of going this route myself.