I typically wait until a refresh and buy right after that. Usually buy every four years. My mid 2010 is still going strong, but that's thanks to some mods and upgrades. Which wouldn't be possible with the new chassis
However like you said, get the best you can afford. No sense spending less money on something you'll replace twice as often. And also think about C4D integration coming with CS7. You're going to want a good mobile graphics card, if you're into that.