Um?! Maybe using a FrameBuffer device rather than X or the Ubuntu desktop would (or rather will) release a lot of steam for your processing of signals.
 
With regards to suggestions to how to interface extra ADC channels to your Gumstix folks would need to know some thing like:
 
How many channels you require?
How many bits per sample ?
How many samples per second will you be making?
Are there any particular signal conditioning requirements you have? ..Buffer amps etc etc!
 
In practice an SPI channel would be flooded by say a single channel of 8 bit samples @ a megasample or two!
 
As ususal more questions than answers ;)
 
John A
--- On Wed, 10/8/11, AKS <aungkyas@gmail.com> wrote:

From: AKS <aungkyas@gmail.com>
Subject: Re: [Gumstix-users] When will new Gumstix board with 1GHz CPU and 1GB RAM?
To: emails081-gumstix@yahoo.co.uk
Cc: "General mailing list for gumstix users." <gumstix-users@lists.sourceforge.net>
Date: Wednesday, 10 August, 2011, 12:46

Hi John

I am running Ubuntu Desktop GUI on Gumstix,
also building an app that is alike an oscilloscope
but with more channels. Cheers!