The Synology comes factory default with 1GB RAM and has a spare 200-pin slot available for expansion. There has been sofar no confirmation what DIMMs work, but I can assure you that if you purchase a 2GB 200-pin 800Mhz DDR2 laptop DIMM it will work.
I bought a relatively cheap Kingston 2GB PC2-6400 CL5 200-pin 800Mhz DDR2 DIMM (KVR800D2S5/2G). I am pretty certain that any other DDR2 DIMM @ 800Mhz will work although you should make sure that it is CL5 and 5-5-5.
Proof:
MuffinStation> cat /proc/meminfo
MemTotal: 3106348 kB
MemFree: 2938772 kB
Buffers: 5320 kB
Cached: 78676 kB
…
