Re: What synth box thingy to go with said synth guitar?
First, do not buy anything unless you understand your needs and how midi works...you will get very frustrated- there is a great Yahoo group (midiguitar) with lots of info.
The Roland floor boxes are good, cheap ways to getting into synth. Downside is that it is on the floor, you step on it, and is more likely to have something go wrong. Also, and this is the biggest- the sounds have little programability and don't really compare to rack synths due to limited sample memory. In other word, the sounds, um, aren't that good. And you can't add any more. The soundset is fixed.
I recommend a Roland GI-20 (I use a GI-10) and synth of your choice. I use a ROland XV-5050. The Axon converter is good, but expensive. It also comes with a cheesy sounding synth built in. Stick with Roland. Now after you have the synth and midi interface, you need a rack to put it in. Add some $$ for a midi pedal to control it - I use a Behringer FCB1010. Oh, don't even think of running the synth sounds through a guitar amp. Add some $$ for a keyboard amp to bring to gigs too- or just run it through the PA. This all assumes you have a midi capable guitar.
This is a serious commitment in time and money. Even once you get it all set up, you gotta program, and learn to play- you can't play guitar licks using a piano or sax sound. It is all in the phrasing if you are planning to emulate other instruments. Learn their ranges and how they make sound. Good luck!