I bought the 512 MB stick from Mushkin and it works fine so far. Also the price differences among the memory is if it is buffered or unbuffere, ECC, performance or basic.
I took this from the Mushkin website hope it helps
"DDR (double data rate) SDRAM is the latest evolution in random access memory (RAM) technology. All SDRAM operates on a system frequency or "bus" speed measured in mega hertz (MHz). Traditional SDRAM used only the rising edge of the clock cycle to transfer data. The new DDR SDRAM transferrs data on both the rising and falling edge of the clock. PC2100 is the common name for the JEDEC specification for DDR which runs on a 133MHz bus speed (266MHz double data rate). The chips themselves are named in as more conventional way as "DDR-233" where the name reflects their MHz speed, the modules however are named for the effective data transfer of 2100 MB/sec (2.1 GB/sec) on a 64-bit bus.
By combining the finest chips, printed circuit boards (PCBs) and our Mushkin heat spreader, these modules insure super fast performance and extreme stability even in the most demanding settings. PC2100 DDR uses either DDR-266a or DDR-266b chips. DDR-266b chips are rated for CAS2.5 at 266MHz operation. The DDR-266a chips are the fastest bin rated for CAS2 operation at 266MHz, we use these on our High Performance DDR SDRAM. These modules are programed for CAS 2-2-2 operation (except for the 512mb High Perf which is CAS 2-3-3) at 266MHz via the serial presence detect (SPD) EEPROM. PC2100 DDR is designed for use with CPUs which run on a 133MHz FSB (266MHz DDR). Although the increase in bus speed from 200MHz to 266MHz does translate to an increase in performance, a much larger gain comes from using DDR which runs at lower latencies. For this reason the High Performance DDR is our best DDR module available. By running at 266MHz at CAS 2-2-2 (CAS 2-3-3 with the 512mb version) this is the fastest, most stable module available. "