UDIMM Ram is same as SD-RAM?
No. DIMM is merely a form factor.
DIMMs come with a variety of memory on them, the most common ones are:
- SDRAM of type DDR3, unbufffered. 64 bit wide.
- SDRAM of type DDR3, bufffered / registered (both names mean the same thing)
- SDRAM of type DDR3, unbufffered. 72 bit wide (64 bit + ECC)
- SDRAM of type DDR3, bufffered / registered, 72 bit wide (64 bit + ECC)
The last is mostly used in servers, the first in consumer devices.
There is slightly more to it, since there are also low voltage versions, and DIMMs can have different ranks. Usually you do not need to worry about this and can just buy the cheapest (often rank 2) modules.
If your server has multiple CPU's (as in, actual multiple chips in multiple sockets, not one CPU with multiple cores) then you might need to fill the memory banks on both CPU's.
If you want to enable multiple memory channels (e.g. dual-channel, triple channel, quad channel) for a slight speed boost (roughly 5% for dual channel), then you will need to use similar modules on the same memory channel.
For any more concrete answers you will need to add more details to your post. Such as which server model. Which CPU(s) are used. How are the memory banks filled? What is the goal? (e.g. add more RAM, add fastest RAM, cheapest ways to add RAM, ...)