Crucial has apparently EOLed that particular kit.
The description was:
Crucial Ballistix 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model BL2KIT25664AA80A
So the important points are:
1) DDR2 800 (or higher) memory in 240 pin DIMM format, unbuffered, non-ECC
2) 2 DIMMs of 2 Gygabytes each, for a total of 4 Gbytes
3) The BL2KIT25664AA80A was rated up to 2.0 volts, but that's for overclocking which the Readynas does not do AFAIK. Any 1.8 Volts kit should work too.
A new parametric search at Crucial returns 2 possible kits:http://www.crucial.com/uk/store/listmod ... /list.html
Any of them should be fine, but try at your risk.