Best place for guaranteed-compatible RAM:

http://www.crucial.com/store/listparts.aspx?model=Dimension%209150

Presumably you meant the 9150;  there is no 9159.

After finding the MHZ frequency your motherboard supports and the type of ram your computer uses I would go with whatever g. skill ram is on sale on newegg.

2x1gb or 2x2gb should be plenty and if I'm correct in assuming your computer is 240-pin then g skill pretty well corners the market on affordable, reliable 800mhz ram.

First of all, I apologise for not noticing the typing error - yes, I meant to say Dimension 9150.

I did explain that I know very little about computers, so the information in the link you gave me does not really help a lot. It seems to be saying that sometimes you do not need to instal two identical bits of memory , but other times you do. I do not know which option applies to my particular computer, or whether I have to remove the present memory and replace it with two new units. I do not know how much memory I should instal, or whether installing it is someting I could do myself. Basically, I am afraid I still am no wiser at all than before I posted my question................

Oh well, never mind.......

For optimum performance, you must install memory in pairs in this system.  You have four sockets so you need not remove the original RAM to upgrade.