:(Although the maximum memory for my DELL dimention 2400 is listed at 2gigs my machine will not boot up with any more than 1gig. I have the model compatible cards from Crucial , is there some changes needed in the bios I should be  making?. The O/S is windows XP  service pk 2.Do you actually have 2Gb of ram installed? Just because that's the maximum it will handle does not mean it has that much installed.I currently have 1 gig installed and  the computer  boots up normally but if try to add any more memory it willnot boot even though Dell says it will take 2gigs. System info recognises the 1 gig .So  i gather the computer up until your decision to increase to 2gb has up to now always had 1 gb and worked properly? That being so you do state " I have the model compatible cards from Crucial " in reference to what  i assume is the additional 1gb ram you PURCHASED. In the event that it is "compatible" as stated by Crucial still leaves the possibility that is NOT in fact the same MANUFACTURER and/or the exact same model as the original 1gb. That can be the basis of your problem. Also does the single "compatible " module WORK on it's own in the computer? If not EITHER it is NOT compatible or is defective. truenorth Dell says 2GB, but they also say modules up to 512MB & it has 2 slots.  Usually, this is because 512 was the largest available at the time.  In your case, it appears that 512 is a real limit.
http://support.dell.com/support/edocs/systems/dim2400/en/sm_en/specs.htm
My Dell Dimension 2400 runs only on 2 512 sticks of RAM. But I would like to test it on a 1.5 or 2 GB if given the chance to acquire another 1 GB DDR RAM. They are quite expensive nowadays and hard to find. Since its just running on XP for light tasking, it runs better today compared before on a 512 module. Having the same computer model, I have seen threads online that have used a total of 2 GB max of RAM on that dell machine. Just a thought on how I added the other 512 stick. The lone 512 stick was on the 2nd slot and when I added the other 512, it makes a beep code/message for the changes or new hardware installed. Making it unbootable.  Removing/Clearing the CMOS battery and swapping places the two RAM on slots went smooth after.