[Veltliner]

For the 24" iMac Core 2 Duo 2.16 ghz. One costs 50% more than the other.

Both from Crucial.

Memory upgrades from Crucial.com - Search Categories

1.

2048MB
CT582930 Apple iMac 2.16GHz Intel Core 2 Duo (24-inch)
DDR2 PC2-5300 • CL=5 • Unbuffered • NON-ECC • DDR2-667 • 1.8V •
128Meg x 64

2.

2048MB
CT612721 Apple iMac 2.16GHz Intel Core 2 Duo (24-inch)
DDR2 PC2-5300 • CL=5 • Unbuffered • NON-ECC • DDR2-667 • 1.8V •
256Meg x 64

The difference obviously is: one is 128Meg x 64, the other 256 meg x 64.

What does this 128, resp. 256meg mean? It can't be buffer, as the RAm is unbuffered.

[mduell]

128/256Mb is the size of each chip on the memory module. Higher density chips are generally more expensive. Take the cheaper one.

[Veltliner]

Thanks!

Originally Posted by mduell 

128/256Mb is the size of each chip on the memory module. Higher density chips are generally more expensive. Take the cheaper one.

Does that mean the 256meg so-dimm has a smaller chip due to density, but they are both equally fast?

Why would a company put a higher density chip on a so-dimm, when it is more expensive?

[mduell]

I didn't look closely enough last time*. I'm not sure how you got to that page you linked to, but it's misleading! The page you should have arrived at after using the Crucial Memory Advisor tool is Computer memory upgrades for Apple iMac 2.16GHz Intel Core 2 Duo (24-inch) Desktop/PC from Crucial.com
It has both parts, but with the very important distinction:
2GB kit (1GBx2) CT582930 DDR2 PC2-5300 • CL=5 • Unbuffered • NON-ECC • DDR2-667 • 1.8V • 128Meg x 64 US $101.99
2GB CT612721 DDR2 PC2-5300 • CL=5 • Unbuffered • NON-ECC • DDR2-667 • 1.8V • 256Meg x 64 US $159.99

Somehow the "kit (1GBx2) text was dropped from the page you got to. You, of course, want the more expensive one.

* 128Meg x 64 is only 1GB; it should be 128Meg x 128 if it was 2GB.

Originally Posted by Veltliner 

Does that mean the 256meg so-dimm has a smaller chip due to density, but they are both equally fast?

Why would a company put a higher density chip on a so-dimm, when it is more expensive?

By density I mean bits/chip, not anything to do with size (although there are half as many chips). The performance is the same.
Some computers/chipsets require particular memory densities, so they produce SODIMMs with both. But it surprises me they'd put both on the system-specific list.

[Veltliner]

Originally Posted by mduell 

You, of course, want the more expensive one.

Thanks.

(I arrived at that other page by typing "iMac 24" core 2 Duo 2.16ghz" into the crucial.com search window.)