Ultra 4 + 4 gig

I'm speaking from my personal experience with PC's now, and not with the ReadyNAS, having said that - the ReadyNAS is a mini computer in it's own right, so you can read into this what you will.

In my opinion, the most critical factors to a stick of memory "working" or not is usually the rated speed it supports and the CAS timings at that rated speed. You may have a stick of memory whose default rated speed is 3200, at a CAS level of 3.5. But putting it in a system that does a maximum of 2700 and requires only CAS 2.5, it may work at the reduced speed with the improved latency. Sometimes manufacturers specify multiple values on the specs, sometimes they don't. I wouldn't bet against the number of chips on a stick of memory affecting the latency, and the other lesser known timings like Command rate, etc will also have a bearing. But in my experience the speed and CAS timings are the most critical.

At the end of the day, the NAS houses your data. Only you can judge if it's worth the risk trying a stick of memory not on the HCL. But if you buy a stick that conforms to the basic specs the NAS requires, as long as it's memory from a trusted source you may get lucky. I do it, as do others on here but it's a practice that is not without it's risks.