There are shed load of different types of memory sticks.
some have a different socket, so it wont actually plug in if it's the wrong one.
Other share the plug type, but wont work.
I personally think the best bet is to phone/E-mail the details of your system, and say that you want memory to work in it.
Only say 'i want memory that works in my PC (details as given), and is XXXMb in size'
If it dont work, they have got it wrong.
If you ask for a 256Mb PC2100 stick, made by 'A Company', and it dont work, thats it not their problem. The sent exactly what you asked for. It's your fault for asking for the wrong thing.
COnsumer laws not withstanding.