|"I have a northgate Computer model AWRADACPI"|
That is not the model number, it appears to be the BIOS type used by the motherboard. Also, Northgate has been out of business for at least 6 yrs.
512MB isn't very much RAM these days, but it should be enough to run XP reasonably well - it depends on what else you have installed. To upgrade the RAM, you need to identify your motherboard & the type/amount of RAM it supports. For an AXP 2500+ you should be running DDR333.
If you don't know the make/model of your board, try using Speccy: