There's no real ideal RMS. If you are amping your speakers then you will want to, for example, buy a 2x100 watt amp for a set of 100 watt speakers, then set the gains on the amp correctly. The gain is basically your master volume control on the amp. You want to turn the head unit to as loud as it it can go, then turn it down a small ammount, then set the gain so that the speakers are not crackling at this volume. Thats the best initial setup, then they can be fine tuned to get a decent sound stage.
If you are running the speakers off of a head unit you don't need to worry. A lot of head unit amps will churn out about 45 watts. This is plenty to run a set of comps off of as most are rated at around 120 watts. Don't worry about the number of watts written on the box though, you don't need to put that much power through them to get them to run...