If you're intor gaming and other work related applications then you can get them value rams, like Corsair ValueSelect rams. That should be good enough for you, as I have tried this with my system before. Getting the XMS2 rams over the ValueSelect rams will give a little improvement in gaming. So the XMS2 are highly advise to be use only for overclocking as I am using right now.
So if you're not into overclocking, then a 2x1Gb of Corsair or any compatible value rams would be good for you. Don't get them XMS2 for non-overclocking for it only gives a few increase, unlesss overclocking.
ECC Registered RAM is not usually known in the consumer or computer-enthusiast division. ECC Registered RAM is a special type of RAM reserved for server or workstation systems. For a desktop or laptop PC, regular unbuffered RAM (or just RAM) is the norm.
So what is the difference between regular RAM (unbuffered) and ECC Registered RAM?
First of all, ECC stands for Error Checking and Correcting. This means that the ECC RAM module can detect and correct any errors found during processing, be it single-bit or multi-bit. In a processing cycle, the computer fetches data (or instruction), interprets it, executes and then stores information in the memory. During this process, soft errors normally occur. (Why does your computer suddenly freeze or hang and requires a reboot?) ECC RAM is capable of checking and correcting any of these errors. For a server system that is kept online 24/7, this is very important.
Second, Registered RAM has extra chips that collect and buffer address or control signals from the chipset (motherboard). In other words, the memory controller does not see the chips in each DIMM slot; it only sees the register. This in turn will lighten the load on the control module.
In contrast, in an unbuffered RAM module, the address and control signals go directly to every RAM chip; the chipset actually "sees" the entire RAM, spread across the memory bank. Some argue that unbuffered RAMs provide a faster access, because data transport does not have to wait until the end of each cycle. Some believe to the contrary.
Actually, back to the Registered RAM, because of the decrease in the load, the chipset in the motherboard is able to access and send address or control signals much more quickly and remain stable.
Just a side note: Some systems only require Registered RAM and some ECC RAM. Nowadays, these two features come in one package. Therefore, if you buy Registered RAM, it is likely that you get ECC and Registered features.
Again, ECC-registered rams is mainly used in servers and other mission critical applications for security and stability. For regular and gaming rams are non-ECC unregistered rams for faster performance.