Building rackmount pc for live use...

Ok im looking for a little bit of info on what to use for this next build.
It will be used for live, mainly playing our sequenced tracks which tbh wont be that resource hungry.

It will be housed in a rackmount case with 120mm fans and a tagan modular or OCZ psu.
Will be looking at 2gb of ram but the main question is.....
What bloody processor????

Me being me i always end up blowing cash on something i dont really need and tbh i was thinking instead of going for a Q6600 slacr, i would get a dual core instead. So what should i be looking at?

The main things it will be running is Reason 4, Ableton Live, Sonar and some softsynths......

This will be a master pc which will be running a laptop as a slave for our keyboardist.

Any help would be greatly appreciated but i really would like to keep the costs down as much as poss and buying 2nd hand is an option :D

Thankyou in advance for any help