You will want to get another stick of that same stuff then. RAM nowadays is pretty sensitive to other RAM modules, your best bet is to get another Kingston thats the same, or sell that 512 you have and buy 1 gig of something.
Buffered ram has a more complex IC. It has additional banks on-chip that hold data for 1 cycle before returning it to the CPU. This adds a bit of overhead (read: latency), but greatly improves the reliability of the RAM. Coupled with ECC, it ensures that the chances of an uncorrectable error occuring are hundreds of thousands of times less likely than with standard RAM.
The difference between registered memory and unbuffered memory is the additional hardware to increase consistency and predictability of the address and data tranmission paths however it was made.
Registered memory is addressed by the CPUs or the Controllers through an additional set of registers, the registers remained the same input impedances to the CPUs and to the Controllers however the memory was made, be it Single-Sided or Double-Sided, 4 chips or 10 chips or 50 chips construction.
Unbuffered memory have varied input impedance depending upon their construction, Single-Sided and Double-Sided have widely varied input impedance from each other.
When using them properly within their limits, there is no difference between them whatsoever for using the PC (Gaming, multi-tasking, video editing etc). Simply expect that there is additional latency introduced by add-on registers in trade for address and data tranmission path greater consistency and more predictable quality despite their varied constructions.
If you are an OC'er - Registered memory additional latency caused slight loss of maximum possible data throughput, mobos using registered memory normally were not made with overclocking in mind as that is definitely not a recipe for consistency and predictability.