Probably nothing for the mobo since only certain sizes would work. Processor wise, you probably can't do much either, unless you somehow manage to get your hands on the appropriate Mobile Pentium II or III (depending on the revision of your notebook). Also, if you can get your hands on a stick or two of 256MB SDRAM @ 100 MHz, you can upgrade to a maximum amount of 512MB.
For more Dell specific help, you may want to check out the
Dell Community Forum and possibly verify all of the upgrades that you could actually perform on your machine.
Link to the Inspiron 7500 Service Manual