From Thinkpad wiki (http://www.thinkwiki.org/wiki/Unofficial_maximum_memory_specs)
Note last two lines.

Hope that this helps,

Mark (Who used to have a Thinkpad 240 J)

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ThinkPad 600

 ?             288MB                   ?              ?            

    * 416MB = 256MB low density PC100 SODIMM + 128MB PC66 SODIMM + 32MB PC66 on-board. It matters which SODIMM you put in which slot. This was first reported working on the Thinkpad Mailing List, and it worked error-free for me.

The 600E (2645-8A0) with Bios INET36WW accept two modules of 256MB. The ram modules have 8 chips on each side. That results in 544MB. (RAM Typ: Micron MT16LSDF3264HG-133E4 PC133 CL3 sync). Processor is an Intel PII 366 PE, installed platform is Windows XP Pro with SP3.

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Sign hanging in Einstein's office at Princeton:  Not everything that counts can be counted, and not everything that can be counted counts.