I have very little knowledge when it comes to midi. I need to get a midi device for Reason/Sibelius, but I also have an ancient Roland JX-3P sitting around. (http://en.wikipedia.org/wiki/Roland_JX-3P) It is an analog, but it is also the first midi-capable synth that Roland produce. It has a midi bus input and output. With the right cable/adaptors/etc., would I be able to use this synth for my computer programs?

Part II: If so, what cables/devices/adaptors would I need?

I don't see why you wouldn't be able to use it.
What sort of soundcard do you have?

I have used on as a midi controller and slave with no problem. It is old and won't take full advantage of the range of controlls available for modern midi software. For note on/ note off /velocity and maybe pitch baend, it would work. But modern stuff like the M-audio 02 series will give you lots of knobs and other stuff to assign as well.

I don't see why you wouldn't be able to use it.
What sort of soundcard do you have?

a cheap Soundblaster live

If you have USB on your computer you can use a USB-MIDI inerface like this:
http://www.m-audio.com/images/global/product_pics/big/uno.jpg

or you can get one of these older ones that hooks up to your Soundblaster card:
http://www.allansmusic.com.au/images/Products/SB8.jpg