I upgraded my PC last night. Only one problem: I installed a faster procsessor on a faster system bus. Accordingly, I installed new RAM at the appropriate bus speed. I put in 4 GB of RAM on 4 gig sticks. My mobo has 4 RAM slots that are all supposed to be able to handle a gig a piece. I run XP Pro, which is also supposed to be able to handle 4 gigs.

So why does my system say that there is only 3 GB??

Any input is much appeciated.

Since you are sure that your motherboard can handle 4 gig, I would suspect faulty RAM or faulty RAM slots.
You could check for that by removing and replacing the rom one by one in each slot and seeing if one slot or RAM constantly fails.

Does the bios show 4 gig?  If not then I would pull each stick untill you find either a bad slot or a bad stick.  It is also possible that the bios just needs upgraded.

ETA... I was a little late.

There are some mobos that just don't "like" filling all the 4 DIMM slots with RAM.  I have seen instances on Anandtech's forums where they mention a problem similar to yours.   Basically even though you've got the 4 slots filled, it won't recognize all the memory modules.   The recommendation for these mobos is to only use the 2 slots.  I'd run a search in those forums to see if your mobo is one of them.

Also, there are some mobos which will take memory in all 4 slots but for some reason won't list all the memory on the BIOS page.  The machine still has (and uses) all of your new memory, it just doesn't show up.  I think my Asus A8N SLI's manual said something about this being a possibility with my mobo (I only have 1 GB in 2 DIMM slots).

Come to think of it, WinXP with service pack 2 has some error in correctly recognising over 3 Gigs of RAM.
Not sure, but I think that it limits to only 3 gigs to stop some problems.
Maybe you could find something about your problem on Microsoft's homepage.

Windows XP will not recognize more than 3GB of physical ram.