You can use EITHER buffered/registered (RDIMM) or unbuffered (UDIMM), but you cannot mix them.  So, if you want to move from UDIMM's to RDIMM's, you will have to replace all existing memory with RDIMM's.  If you choose to stick with your UDIMM's, then know that 2GB is the largest UDIMM you can use.  If you want/need to use larger DIMM's, then you will have to replace the memory with RDIMM's. Do you have one processor or two? If one processor, then you can only use 6 DIMM's, giving you a max of 12GB if using UDIMM's.  If you have two processors, you can put 2GB in each slot, giving you 24GB total. It doesn't have to be "Dell" memory, but it does have to meet certain criteria.  If you don't purchase from Dell or someplace like Crucial that tells you what you  need and guarantees compatibility, then make sure you review the population guidelines (page 30): [URL]  
I bought this a few years ago, and it seemed OK initially... After restarting it's used perhaps 3.5GB. After a bit of time it's up to something near 5.8GB and lagging significantly. This is the memory which my profile shows is installed: > DUAL IN-LINE MEMORY MODULE, 2G, 1333, 2RX8X72, 8, 240, UBE Processor (1): > E5520, 2.26/5.86, 8MB, XDN, D0 Obviously, three of those. I'm thinking that bringing it up to double or triple that would give me some headroom. I"m running SBS2008, and not much else except a dental practice management program. The memory was "unbuffered", but I really have no idea why that was picked for the install.  Is it better to add memory to what I've got (would have to be the same?) or take it out and go with perhaps 3x8GB's ?? Buffered.. unbuffered??? Does it matter if it's DELL memory??  THANKS 
T610 - 6GB is NOT sufficient!