Wow, nice old mobo, i am glad to see that its running fine since you purchased it, i am not surprised just took alook at its specification and VRM and cooling :
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131361
No i dont think thats what they mean when they said that :
"Due to AMD CPU limitation, DDR2 1066 is supported by AM2+ / AM3 CPU for one DIMM per channel only."
i reckon they meant NO dual rank memory in one slot, see in order to cheaping the cost of RAM, manafacturers, used to pull of something similar to RAID 0 in ram, especially in current 8 GB of RAM, they would dual stack them, meaning the ram dimm is supposed to by defualt to be 4 GB but they added another 4 GB at the other side, thus its called Dual Rank, sometimes those rual rank RAM causes issues especially with Asrock mobo`s back in the day.
What you need is single rank RAM, which is more stable and faster, the limit for single rank today is 4 GB per RAM dimm anything higher is dual rank, however there is 4 GB dual rank
I did quick serach and sadly i am not able to find out easily single rank DDR2, you need to find them your sef sadly:
for ex this is dual rank L:
http://www.kingston.com/dataSheets/KVR800D2N6_2G.pdf
i recokn its not about the size and filling the 4 slots on the mobo, you can fill them all, just use single rank RAM
just my 2 cents
today iam sleep deprived so i am sorry for the typos