Triple channel ddr3 ram

On the vast majority of motherboards which have dual channel capability, installing an "odd" number of modules will result in dual channel being turned off entirely. So it will result in 3GB of single channel most likely. There may be some boards that won't like having 3 sticks enough to fail to boot - but they allow allow a single stick.

All motherboards I'm aware of will allow 4 sticks of identical 1GB RAM to run as full dual-channel memory.

There are some boards and cpu combinations, I believe limited to AMD, that won't run the modules at full speed if there are 4 rather than 2 of them. But even then, dual channel works.

It appears from my reading and comments here that if you use 2 sticks of RAM rather than 4 the system uses it more efficiently. In addition, with 4 sticks you sometimes need to increase the supplied voltage a bit more than with 2 sticks to keep all 4 running at spec.

Be sure that for an i5 system you choose RAM that uses between 1.5V and 1.65V. This is true of any triple kits, but make sure it doesn't say more than 1.65V for any single or dual kits. Many AMD boards will use higher voltage - which is why you see some higher voltage modules out there. This is usually the only difference between "i5/i7 compatible" and modules that don't say this. And don't worry if the RAM you like doesn't say i5 comp if it runs at 1.5V-1.65V.

I think you'll find that its cheaper to get 4GBs as 2 - 2GB modules rather than 4 - 1GBs. So if it's cheaper and works even a little better, go that way.