First, you should explore how much Data (pix, music files, letters to mom, etc.) you've got stored on your current C: Drive. Those CAN be eligible for removal to a new 2nd hard-drive (or 3rd or whatever).
If you're using Windows (XP? Vista? W7?), I can right-click on the MY DOCUMENTS icon on my desktop and 'move' all of those files to a new, more-empty HDD, thus emptying C: Drive a bit more.
(The MY DOCUMENTS settings are not too thorough, though - plenty of 'settings' files are still sadly stored on any C: Drive. This Migration-Move ends up putting some of your changeable Data Files on one drive, and some - like Favorites-Bookmarks, Desktop icons-files - remain on C: Drive. Grrr...)
I am killing time for the moment, awaiting a morning-fishing expedition to assemble.
I was looking at LogicBuy's Hard Drives ("Lowest Price first" sequence) page and saw some incredible deals - INCLUDING some pretty great deals on SSDs.
There's a 120Gb OCZ Agility3 for $69.99. A Seagate Barricuda 1Tb HDD for $79.99 (which will have 'cloning software utils' available on the Seagate website that merrily will clone some other mfr's HDD onto their Seagate products).
Then, in the $165 range, there are some 256Gb SSDs being offered.
I still think "A couple of small SSDs PLUS a lot of good User Discipline" is the better behavior, but I know if I had a BIG SSD, I'd probably fill it up, too! Darn...
And finally, maybe I should comment: "Reloading Windows IS inevitable" in my opinion for ALL computers. It's actually a good thing, though. When I do wipeout's-and-reloads, my computer LOVES it.
For a while. Then I gunk it up again.
This Wipeout-And-Reload mentality forces me into several disciplined behaviors, though: (1) Backup all data; (2) maintain copies of all programs I use and want to re-install; (3) be prepared to call Microsoft when they refuse to activate my Win License and this is usually a 6-minute touch-tone phone "conversation" between me and some robot.
Oh. And finally... accepting that I'll screw this up and lose something. Something! After doing it a couple of times in a year, though, whatever I've lost is either long gone and nearly forgotten, or else I've found it and will be more careful. Next time. There's always a next time.
So, YES I consider reloading Windows to be inevitable for all computers.
But NO, not necessarily in THIS case. If you've got backups and have access to all wanted programs, then you have a great luxury of choosing your computer's future. (Personally, I don't think you're going to get much 'bang-fer-yer-buck' improvements over any new hardware. It will be prettier, shinier, but I doubt if you'll see 5% performance improvement for a LOT of new dollars spent.)