The model number will be crucial, since each model can use different motherboards and in fact there are even Compaq Presario desktop and notebook models available with that processor, which you also never specified, though I'm betting it's a desktop.
The Presario desktop models of that era appear to have shipped with 96MB RAM, so the 4MB missing is probably reserved for the video memory.
They also indicate that the 3 DIMM slots are upgradable to 384MB (SyncDRAM DIMM required). This might be proprietary memory, but we'll know better when you provide the model number.
Looking up the RAM for the desktop models I found using that processor shows modules up to 64MB each available from one vendor, which would provide 192MB total, less the 4 to 8MB used by the video. This might barely be enough to operate, though it would be better if you could find a couple 128MB modules somewhere.
Bitman