The basic specifications say 256 MB is the max: www-307.ibm.com/pc/support/site.wss/MIGR-4BW6R2.html, using two 128 MB SO-DIMMs.

However, the Memory options for the “X” series shows that, unlike the base models, it accepts 256MB SO-DIMMS, as well as 128 MB SO-DIMMS: www-307.ibm.com/pc/support/site.wss/MIGR-4BXPQ3.html. So if you put 256 MB SO-DIMMS in both slots, it appears your system could be upgraded to 512 MB. The required part appears to be a standard 144-pin SO-DIMM: www-307.ibm.com/pc/support/site.wss/document.do.

You haven’t mentioned whether your current 256 MB consists of two 128 MB SO-DIMMs — one in each slot — or whether you have a single slot with a 256 MB part. I suspect it is the former, in which case you will need to remove both 128 MB parts, and replace them both with new 256 MB parts. If you have the latter setup, you will only need to buy one new 256 MB part, and insert it in the extra slot.

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Disclaimer: The information presented here is only as good as the accuracy of the information on Lenovo’s web site. I have not personally tried upgrading this unit. Do not buy anything unless you can return it, in case the upgrade does not work. If an attempted upgrade does any damage to your system, I’m not responsible. Your mileage may vary. Sparks and flames are cool to watch, but bad for your computer.

Notebook IBM Thinkpad 390x