can I also use something like this -
4GB (4x1GB) Corsair PC2100 Reg/ECC DDR RAM ?
As dc3 says, to be sure we will need details of the board it is to go into.
Generally speaking, memory specified this way is probably intended for use in a server, and most desktop motherboards would not be able to use it.
ECC RAM has extra memory bits available for Error Checking and Correcting - a board which does not need these because it does not do ECC may just ignore the extra bits, but equally may find it confusing and not be able to boot with the ECC memory fitted.
"Reg" indicates registered memory - again designed for servers with large amounts of memory. Each DIMM having a register reduces the load on the memory controller so it can drive a large number of DIMMs, and increases the integrity of data operations, at the expense of speed because an extra cycle is needed to transfer data through the register.
Memory that is both registered and ECC is likely to require a server class motherboard.