I've got JME application that successfully runs on a PC with a dedicate graphics card. The same application, with identical JVM settings and RAM but with onboard graphics will not even create the game window.
WindowsXP
JRE 1.6 update 10
JVM Setting
A pixel format exception would be thrown if the graphics driver can't handle the resolution/bit depth combination jME is trying to use. It shouldn't have anything to do with your heap settings. Try starting your app using a basic setting of 640x480 @ 24, see if it comes up.
I had a similar problem because I used multisampling and second computer didn't support it. Try with sample = 0 ?
The SimpleGame application with 640x480 @ 24 works, however when I include the heap setting of a gig the window does not open and the following stack occurs
org.lwjgl.LWJGLException: Could not find a valid pixel format