my apologies if I misuderstood.
The overall basics that one should look into when considering most efficient RAM chips:
DDR2 chips draw less volts (c. 1.8V) than DDR (c. 2.8+/-3).
Overclocking chips, esp DDR are almost always high voltage chips (up-to 3.5V if your mobo can give that).
The optimal voltage of each DIMM is often based on the chip type used. As discussed above, old Winbond BH-5/CH-5 and the newer UTT are very volt hungry, esp. if you want to overclock them and try tight timing.
TCCD series chips from Samsung usually get by with much less voltage, hence no need to overvolt the DIMMs as much as with Winbond/UTT.
The number of chips is also important: the more chips you have, the bigger the current draw (i.e. bigger memory size cause higher wattage thermal dissipation).
More voltage and more current mean more wattage draw = more heat, more need to cool the dimms and the system.
These are the basics to consider
It is very hard to give exact recommendations for makers/models as these sometimes also depend on the motherboard and/or cpu type used.
Also, as stated above, for A64 platform super-tight timings aren't that important anymore. The difference between CL2 and CL3 (CAS) is very small in most everyday tasks/games. Only if you are serious about benchmarking, does it really come into play.
With this in mind, it'd be best to try and find the highest quality TCCD chip based (pref. with brainpower pcb on the DIMMs), with no more memory than you need and try to use them at nominal voltage or below that.
For a list of TCCD based DIMMs, see:
http://www.xtremesystems.org/forums/arc ... 43659.html
http://www.xtremesystems.org/forums/sho ... hp?t=50010
Again, xtremesystems.org is the best source in these matters.