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
