Any high performance laptop + probably a firewire audio interface and a midi controller would probably be ok. Are you planning on using this for recording as well?
When you say performance, are you meaning in terms of as a basis for your soft synths/real time sample triggering etc?
For purpose built machines (though a little pricey) check out:
http://www.sub.co.uk
http://www.dv247.com/dvpc
Otherwise, you're probably best of with a decent regular pc supplier, novatech are pretty good in all the time I've dealt with them, but I'm sure there are plenty of other comparable places around
http://www.novatech.co.uk