There's no way you could PAY me to go back to PS7 or lower because there's too many features I depend on (Healing Brush for example). THAT being said, if 5.5 does everything you need it to, why upgrade?

Yes, Photoshop is a resources hog, but so was 5.5 when I was using it (granted, on a P3 with a whopping 512MB or RAM and an astoundingly big 13GB hard drive - oh yeah, living the fast life - ya jealous?). Given today's systems, it kind of balances out really.

Good luck-
-Lew ;-)