An upgrade is possible, just not an in-place upgrade like one can perform going from Windows Vista to Windows 7. Coming from Windows 2000, you would have to do a migration install (the same thing you would do if upgrading from Windows XP), but if you're coming from Windows 2000, you might as well back up your data, wipe the drive out and do a clean install of 7.
I think the first response from The_Prophecy is the best solution, it would also be the cheapest, $104.99 at Newegg.
You have a new build that you would have find drivers for W 2000, may as well go directly to W7.
Is it possible to upgrade from windows 2000 to windows 7? I just finished my first build and I have my old windows 2000 disk. Can I just purchase the windows 7 upgrade version?
It's not possible to do a direct upgrade (what Microsoft call an 'In Place Upgrade') so that all of your programs, data, and settings are preserved. For licencing purposes, though, it is allowed to use your Win 2K key during a CLEAN install as the one you are upgrading from.