I usually only buy a completely new system when it's no longer cost effective to upgrade the one I'm currently using and when that system will not run current (supported) software. There are fairly well established price points at around $1K, $2K, and $3K (including service/support contracts for a fully equipped system with monitor, etc.), with the $1K system being sort of entry level and the $3K being the Uber-nerd lust object. I find the middle price point to have a reasonably useful lifespan (thanks in part to being upgradable to bigger drives, more memory), but as with many things, your mileage or requirements may vary.