To add to the discussion, OHMS is the scientific term to describe resistance. The less resistance in the circuit the higher amount of power can be pushed through that circuit. Think of it in terms of a hose and water; a 3" diameter hose will allow more water to pass through it then a 1" diameter hose. In the previous example the hose represents resistance (OHMS) and the water represents power (WATTS).
In car audio we typically have power issues as well as space issues. Therefore, factory designers tend to lean towards a 2 ohm configuration because it can generate more wattage from the same amperage in about half the physical size. For example, take a look at the Jeep's Infinity, 8-channel, 368 watt amp. What you will find is an amp that is smaller then a typical hard back book while a comparable 4 ohm amp is about the size of a typical box of cereal or larger.
Another thing to consider with resistance is that the lower the value the higher the noise factor. Typically speaking, an 8 ohm amp is quieter then a 4 ohm amp which is quieter then a 2 ohm amp. Even though less resistance allows for an increase in power it comes at a price which is a noisier audio signal. This is why home audio is typically designed around an 8 ohm configuration. Regardless, this extra noise is mostly inaudible while driving in a car because the vehicle's ambient sounds either over power it or cancel it out. This is another reason why car audio designers don't worry too much about a 2 ohm system.
So, what's the real world difference between 2 ohm and 4 ohm load in relation to stereos? Nothing really. If the system is configured properly then either resistance will net favorable real world results. It's when people start mixing and matching different devices with different specs that things get confusing. If you take 4 ohm speakers and hook them up to a 2 ohm amp then you open the door to other factors that can hamper the results you're looking for. The best thing to do is keep within the specs of the factory system if you're just upgrading components which will net the best results. For example, the factory system is pushing power at 2 ohms. If you want to upgrade your speakers then make sure you get 2 ohm speakers or if you want to upgrade your amp make sure it can operate at a 2 ohm load.
One last thing to consider when upgrading any stereo is the type of enclosures you're putting those fancy, top of the line speakers into. I tend to think of speaker enclosures as the red headed step child in stereo design because most people don't give the enclosures the necessary attention they deserve. In the Jeep the enclosures are made out of abs plastic that resonate and buzz when enough power is pushed to the speaker. This resonance colors the sound output of the speaker and robs the system of power resulting in the user getting upset because their $1,000 stereo sounds like crap.
If you invest money into your stereo make sure you address any weaknesses that the factory enclosures may have. In the case of the Jeep I've found that several layers of Dynamat are needed on the enclosures so they don't resonate. In my opinion, deadening the enclosures in the Jeep is the best investment to your stereo you can make.