What the hell man, the computer you already have is absolutely fine for 3D work. From how you described it earlier, I assumed you had some sort of 8 year old 32 bit XP machine with maybe a couple gigs of ram. You should at least try downloading some trial versions of software on your machine and see how it goes.
Don't change or upgrade anything. If you gotta have a cintiq, then whatever - get it I guess if you know you're going to be doing a lot of painting/drawing.
Overclock it only if you have a nice CPU cooler and fans. It’s already a snappy CPU at stock speed though.
Really though, stop worrying so much about the hardware and get busy learning CG already. Even if you only had an 8 year old XP machine with 2 gigs ram, that would have been a top of the line system 12 years ago and you’d still be able to learn a LOT of CG stuff with it until you started getting more advanced and into files that would be too large for an old machine to handle.

