Memory upgrade for ReadyNAS Pro 6

I've gone with eBay.  The latest I bought was G.Skill 2GB PC2-8500U DDR2 1066 Desktop Memory, which is CL5-5-5-15.  Got 4 for $72 about a year ago.  I've also used OCZ Platinum, which has similar specs.  Be careful with the height of the heat spreader.  Anything more than a simple wrap-around like these have will be too much. Finding 4GB modules used is a bit tougher, as most of this came out of XP machines where 4GB was the upper limit.

But I found I got a lot more bang for the buck upgrading the processor to a Intel Core 2 Duo E7500 2.93 GHz / 1066 MHz / 3M socket 775 CPU SLGTE, which now goes for under $10 on eBay.  The original processor in the Pro BE or Pioneer is a Pentium Dual CPU E2160 @ 1.80GHz.  Only caviat here is to make sure you have the latest motherboard BIOS (FLAME6-MB V2.0).  I understand there are some very early models that are incompatible.  Stock fan and heat sink keep it nice and cool (at least using Arctic Silver), as it is still has a 65W TDP.

BTW, your title says "Pro 6" and your text says "Pro Pioneer".  While they are basically the same, they are not exactly.  The base CPU in a Pro 6 is a Pentium Dual-Core E5300  @ 2.60GHz.  Not enough extra bang to bother upgrading that to the Core2 above unless you need virtulalization (VT-x).  BTW, in OS 4.2.x, CPU and motherboard BIOS info are in dmesg.log.  In 6.x, it's not nearly as informative.