When memory (DDR2) is rated at a certain voltage (2.2V), does that mean it must be run at that voltage, or will it run at the standard 1.8V but not meet its high speed specification (4-4-4-12) unless it is at 2.2v?
Will it operate more like typical (5-5-5-15) memory at 1.8V, if that is all the motherboard does.

The application is Patriot DDR2 800 MHz (PC2-6400) memory in a MSI 945GCM5-F V2 motherboard with a Pentium DS 925 CPU.

I don't care about overclocking, just a reasonable and stable machine.

It means it will meet its spec at that voltage. Everything else is up for grabs.

Anyone with experience as to what normally happens?

Every PC memory module (either SDR, DDR or DDR2) MUST work reliably when configured at the standard JEDEC settings, that is, for a PC2-6400: 400MHz (800 is the Double Data Rate in MT/s, not the clock in MHz!), CL5-5-5-15, 1.8v

If you don't play with the BIOS, the mobo should select that settings as per default, but some DIMMs have buggy SPD, so always check the timings.

You are right: the increased voltage is for overclocking only and is needed in order to achieve the specified minimum timings, but if you don't overclock and use the default JEDEC settings you can safely use the standard 1.8v voltage that will reduce power consumption and heat generation and will allow longer life to you memory.

Yes exactly!
The well-calculated, obviously intelligent, shrewd common-sense guess strikes Bull'sEye, restoring my faith
You will see a lot of 5-5-5-15 @ 1.8v and it's not a bad thing per se; it's just that we like high-perf (of course!) and that means 3-4-4-10 @ 2.2v
But! - the boys are running @ 1066MHz now, they're not fookin' around, and memory timings are not so tight.
They pray at the Altar of Bandwidth. This is particularily beneficial in memory bandwidth benchmarks.
I run my E6420 @ 300MHz which is quad-pumped (so they call it FSB1200).
I run my PC2-6400 @ 600DDR, which is 1:1 synchronous, and I concentrate on sharp timings @ low voltages.
I can get sharp timings @ 2.0v (c'mon you gotta give it a little something) when not pushing the FSB:memory ratio.
1:1 floats my boat as memory bandwidth beyond that of the CPU shows less and less benefit.
So my CPU is 300 x 4 = 1200MHz and my memory is 300 x 2 or 600DDR
x2 (dual channel - 128bit memory) gives 1200Mhz, it's all my OC'd CPU needs LoL,
Therefore, even a FSB1333 rig OC'd to 400MHz would need only DDR2-800...
Regards

You guys are great.

All I want to do is get a decent machine out of a bunch of parts that were "CAR" (Cheap After Rebate)!

Next is the video card, but that is for a different forum.