Can't install more ram on my IBM Pentium 4 CPU 1.60ghz

Hi, I have an ibm pentium 4 cpu and I have only 256mb of ram on it. I tried to intsall more ram but it won't accept any other sticks. Stumped.
 
Not enough information.

Which computer do you have?
Which RAM frequencies does your computer support? (DDR 266, DDR 333, DDR 400, etc...)

Which memory module are you trying to use for upgrade?
 
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