Hi, I hope this will help you (from Kingstone memory guide http://www.kingston.com/tools/umg/default.asp)
ECC: (error correction code) method of data integrity checking that can detect and correct
Parity: Parity can detect – but not correct – single-bit errors.
Fewer and fewer PC manufacturers are supporting data integrity checking (ECC) in their designs. This is due to a couple of factors. First, by eliminating support for parity memory, which is more expensive than standard memory, manufacturers can lower the price of their computers. Fortunately, this trend is complemented by the second factor: that is, the increased quality of memory components available from certain manufacturers and, as a result, the relative infrequency of memory errors.
The type of data integrity checking depends on how a given computer system will be used. If the computer is to play a critical role – as a server, for example – then a computer that supports data integrity checking is an ideal choice.
The term CAS latency refers to the number of clock cycles it takes before a column can be addressed on the DRAM chip. Latency is a measure of delay, so a “CL2” CAS latency factor indicates a two-clock cycle delay, and a “CL3” latency factor indicates a three-clock cycle delay. When SDRAM chips first came out, it was difficult to produce chips with a CAS latency factor as low as CL2. And although some specifications called for CL2, many modules worked fine at a CAS latency factor of CL3. (CL 2 is better)
I think you need PC133 (non-ECC) CAS 2. 256 MB Stick is about US$ 30-35