Ubuntu not acknowledging\using all physical memory

The operating system in question is Ubuntu 8.04 Hardy, kernel 2.6.24, desktop environment Gnome 2.22.3. I've got it on an LTSP server and the thing is getting kind of old so I wanted to upgrade memory to try and squeeze a little life out of it. The server physically has 6GB of memory installed, there are 6x 1G sticks with two empty slots. When I boot up the server and go into the BIOS, it shows I have 6144MB of RAM installed (the full 6G). But when I look at the System Monitor in ubuntu it says only 3.5GB of memory, and when I do the top command 3632060k of memory (basically 3.5GB). So the server itself knows how much memory is there, but ubuntu doesn't. I'm not the person who built this server so I don't know what the hell is going on, the thing has lots of weird kinks and problems. Is there some sort of way I can get ubuntu to actually acknowledge and use the full amount of memory?

64-bit Ubuntu ?

Nope, just plain old 32 bit.

64-bit processor in the machine ?

Yes, two dual core Xeon processors. I thought maybe the memory wasn't installed in the right slots, but the label on the lid for the case says to start with slot A1, then B1, then A2 and B2, and so on. Thats how they are installed.

Doesn't a 64-bit processor need a 64-bit operating system to utilize more than 4G of memory ?

Could be, I don't know. So that means that basically I've got two gigs of memory in there that will never do anything? Thats dumb. I do have a new server to replace this one, I'm just waiting to move to it until summer break. spring break is in a few weeks so maybe I can have it ready then. I hate my job.

I have this ominous feeling that you have a brand new server that you don't even need.

So this old system has room for 8G of RAM total ?
Did it have 4G in it when you got it and you put the other 2G in to make the total 6G ?

It would be funny if whoever set the thing up to begin with put 6G in and installed a 32-bit OS, meaning you've only been utilizing 66% of the RAM this whole time. If there was only 4G for starters I can see why they would have installed 32-bit Ubuntu, they probably figured they would be upgrading the entire unit before they would get a chance to upgrade the RAM. I have a 64-bit processor Ubuntu system here that uses the 32-bit OS because the system only has room for 2G of RAM anyways.

It had 6 gigs in it before I started messing with it. I thought it only had 3.5GB because when I looked in system monitor it said so, but I was surprised when I opened it up and saw 6GB. So I had eight 1GB sticks to put in there thinking it would be a huge improvement but things went downhill fast. I'd like to max it out with the 8 gigs just to get me by until I can deploy the new server. 3.5 gigs just doesn't quite cut it when I've got 40 or more people logged in and using open office and the gimp and stuff.

If it makes any difference, the server was bought a couple years ago from some fly by night mom & pop internet store that sells refurbished equipment. So I think the whole thing is probably screwed. The operating system was installed by the guy I replaced and everything he had anything to do with is junk, if it were me I would have stayed far FAR away from the ubuntu. My new server has two of those new 6 core AMDs and 16GB of memory with 1.5T of usable hard drive space, it blows this little piece of junk I'm on now out of the water. I've got RHEL5 on that one.

So its a 32 bit OS and it can't use more than 4GB of memory? That is utterly stupid, I'm sorry, but it just is.

It's a lot like trying to get 4,294,967,295 people to stand in 18,446,744,073,709,551,615 squares.

Nope, its just math lol

I guess I'm just going to sit tight and bite my lip while people complain then. I've got to set up the new LTSP and build the operating system for the thin clients, install about 20 printers throughout the building, and then on a day with no school move over 200 peoples worth of stuff from the old to the new. And then I can test it and see if it works!

That still seems unlikely that only 4 gigs of memory will ever work. I think you guys are yanking my chain.

Nobody's yanking your chain, but:

This installs PAE extensions which will allow you to access your extended RAM.

*edit
looks like they changed what you need for 9.10 - the packages are linux-generic-pae and linux-headers-generic-pae