I purchased 2 Crucial DIMMs from Micro Center for $16.99 (regular label price is $22.99) that are working fine in my Ultra 6. The label says: 1GB 240-PIN DIMM 128Mx64 DDR2PC2-5300 UNBUFF HALOGEN CT12864AA667
I read some posts where some compatibility issues have been caused by the number of chips on a DIMM. The Crucial DIMMS that I bought have 8 chips each, as opposed to the DIMM that comes with the Ultra 6 which has 16 chips. The original DIMM has this on the label: Unigen UG12T6400L8DU-6AP 1GB DDR2 667 UDIMM 240PIN 1028-MU66TDL80016AUCKP MO9907050008BB0373
The chips on the original DIMM are this: Hynix HY5PS12821C
I did some non-rigorous testing and using Bench32 from ATTO I found that it wasn't worth the upgrade, at least for me. I only ran the test twice before downgrading back to the original RAM and twice after (once using "Overlapped I/O" and the other using "Neither"). Some individual measurements were slightly better, some slightly worse, but they were all in the ballpark. I suppose that there could be some improvement in caching heavily used files, but since most of what I do is stream large files (movies) I don't care about caching as much as raw throughput. So I'll use the memory in a new backup PC that I'm now building to back up my ReadyNAS to.