Upgrading Multi-channel memory architecture (www.wikipedia.org)
Dual-channel architecture requires a dual-channel-capable motherboard and two or more DDR, DDR2 SDRAM, or DDR3 SDRAM memory modules. Memory Upgrade for the Dell PowerEdge 2900 III Server takes the PC2-5300 Fully Buffered ECC DDR2 SDRAM DIMMs. You will need to make sure you place 4 of these DIMMs in 2 matched sets. If the sets do not match the server will not accept it, regardless if they are the same size. Here is an example of a DIMM set that does NOT work.(See Image below)
My Two Cents: This DIMM set does not work in the Dell Poweredge 2900 or 2950 because one is a 2Rx8 and the other is a 2Rx4. The DIMMs must be exact. The Poweredge 2900 has 12 DIMM Slots. The server will throw an error message if a group of 4 DIMM are not installed together, it will still boot, but the error is annoying and can effect performance. If the set of DIMMs do not match, the server will NOT boot.