I've been playing with the "Low Memory" option under @Quarx's CM11 and I must reckon I don't understand how it works. Enabling it leaves my MB526 with just 20-30MB of free RAM, while disabling it boasts free RAM to 150-200MB...
Besides, I didn't notice any drastic boost associated with this great amount of free ram (apps are still closed even with that much free ram)
I think there are 2 ways to see it :
Either the "free RAM" indicator isn't actually accurate and is completly bogus.
Or the "low memory" option, when disabled, makes android kill everything very fast because it expects to have 200MB free at any given time (which means 20% on a 1GB system, that wouldn't be unusual).
So, either way, enabling "low memory" would be best, because I don't think that having android kill all background tasks is a good thing.