Computer not fully reading my RAM

[tigerfootball05]

I know the third slot works so that can't be it. I'm looking for a cheap (or free) stick of 184pin ram but I can't spend much on a stick of RAM that may or may not solve the problem

 

[brinks]

Go return your 1GB of ram and get 2x512mb of ram instead.

 

[tigerfootball05]

So i stole a 512mb stick from my brother's desktop to see how that would read. So now i have a total of 2.5GB worth of RAM in my computer, but its only reading 2GB. I'm thinking it will only read the sticks in pairs, thus it is only reading half of my 1GB stick so it can work with the other 512MB stick. I'm not sure what would happen if I bought another 1GB stick and I don't have the disposable funds to find out, so until I decide to upgrade again, i'm just going to use my brother's RAM until he realizes I have it Thanks for all the input!