My RAM was setup in two 512 sticks, I bought two 1gb sticks and when I booted it up, it would just beep at me constantly.
What I did to get it working, as I have four slots for RAM, in pairs of two, I put 512 in the first slot, then 1gb in the second then I did the same in the other two slots, the push down tabs to pop-up the ram where different colours, black and white.
On the first set, it was like this :-
[black tabs] [512]
[white tabs] [1gb]
2nd RAM slots :-
[black tabs] [512]
[white tabs] [1gb]
You could try setting it up like that, but without the 2nd 1gb stick. Either way i got it work after trying loads of different ways :)