A lot would depend on exactly what software you intend to run. Once that's settled, it should be fairly clear what HW specs are required.
For road use you'll want at least two identical rigs if the system is going to be an essential part of your act. If it isn't (i.e. you can do what you need to do on a standalone keyboard if necessary) you can chance going out on the road with only a single rig.
In general. something either very rugged (i.e. a "field-ready" laptop or "industrial PC") or something easy to find, small and straightforward, relatively cheap, and fairly reliable (i.e. a MacMini) is probably a good way to start thinking about it.
I don't like 'real' PCs for most live venues because I tend not to trust CPUs unless they're built into a keyboard or dedicated musical device. I'm also generally not too keen on packing a lot of high-end technology for a gig unless it's part of a show big enough to hire a tech roadie or two. And it's not because I can't handle the technology myself. It's more a matter of me wanting to stay focused on what my real job should be: playing good music and spending some one-on-one time with audience members during breaks.
IMHO too much time spent having to worry about the' tools of the trade' (because they will break at the worst times) distracts from a musician staying focused on the music and the performance. It's ok to do that in the studio if you want to. That's what studio and woodshed time is for. But when it comes to playing for an audience (especially a paying audience) there's no margin for error or stage delays. So simplify, simplify, simplify. Unless you can afford complexity.
But I'm a just bass player who can't even walk and chew gum at the same time - so what do I know. 
-40hz
Agh! As usual, words of wisdom from 40.
Well, yeah, here's the issue. I already have a panasonic toughbook I'm using currently (or rather, I'm experimenting with, I don't trust it yet). But it's not powerful enough to handle the new Atlantis piano module for the TruePianos software, which is a great sounding piano. So I was going to build a new computer specifically for gigging.
Now, the Roland itself has decent sounds. My biggest problem as a keyboard player is I usually can't hear myself play. So if I'm cutting right to the point, I probably need monitor speakers for myself more than anything else. i should worry about this computer setup later. Ok. I need to upgrade my pa audience speaker, and also purchase a set of monitors for myself. I'll do the computer stuff later. I have about a month. If my setup is not good next month, I'll be stressed.