[phs]

I know RAM must be installed in pairs of equal size and from the same manufacturer, but does that mean that ALL the RAM in the computer must be the same size and from the same mfgr?

I have 4 1GB modules in the Mac Pro already (2007) and just bought two 2 GB modules. Can I add the new RAM in the proper location (i.e., pairing in the requisite slots, or must I get rid of the older 1 GB modules?

[bearcatrp]

You can keep it. Just put the 2GB modules in riser A closest to the motherboard, 2x1GB modules fill the rest of riser A and the other 2x1GB modules in riser B closest to the motherboard.

[mduell]

Originally Posted by bearcatrp 

You can keep it. Just put the 2GB modules in riser A closest to the motherboard, 2x1GB modules fill the rest of riser A and the other 2x1GB modules in riser B closest to the motherboard.

2x2GB on one card (in the slots closest to the logic board) and 4x1GB on the other card is a marginally better configuration.