According to Dell, various pundits in support forums, and a lot of online RAM merchants, exceeding 512 MB of RAM in a Dell Inspiron 1000 shouldn't be possible. Nonetheless, several people have been able to install 1 GB PC2700 333 MHz DDR SO-DIMMS in these machines without problems. All it takes is unplugging the machine, flipping it over, removing the battery and two screws, removing any existing SO-DIMM from the expansion slot, and then sticking a new SO-DIMM in its place. This procedure will yield 1.18 GB of usable system RAM. (The rest of the RAM is devoted to the video subsystem.)
Why do this? It's a great way to boost the performance of these old machines at relatively low cost, particularly if you only have 256 MB of RAM. (I only paid around $30 for a 1 GB RAM module made by Samsung.) Having over a gigabyte of RAM makes it feasible to run things like Microsoft Office 2007, or even...Windows 7. Stay tuned!
PS. I'm by no means the first person to upgrade an Inspiron 1000 with a 1 GB module, but there continue to be fairly strident statements from some sources that 512 MB is the absolute maximum amount of RAM that can be seen in these machines (the 256 MB that are soldered into the machine and an additional 256 MB via an expansion slot). Unless there is some rare motherboard variant for which this would be true, it's just not so.