Though I am fairly certain this should be a topic of its own, even if you want to use it for Skyrim

I will answer you:
First of, you can compare CPU's by benchmark tests, such as this - they are graded and should be easy for you to compare:
CPU PASSMARK
So in response to your question the i5-2400 is better by far (and even though this is a benchmark I also believe that to be the case).
Then in regards to RAM: Its always a good idea to have a large amount of them since it allows your computer to do more at the same time (hence faster), but keep in mind that you need different OS (like the difference between 32bit and 64 bit) depending on the RAM if you want to make full use of them.
Check it here though you can also find that information on Windows own site, but figured it was more simplified here.
Then further in regards to RAM you can also consider Hyper ram, meaning that they are 1600 MHz instead of 1333 MHz, hence faster. I would usually recommend using Kingston produced RAM - also make sure you use only the same type of RAM in the setup.
In regards to a graphic card it depends on your computer type, since you haven't said if its a desktop or a laptop your looking for (looking at the CPU I assume a desktop).
But I can say that for the laptops I would recommend something along the line of: GeForce GTX670m or the AMD HD7970m - both easily capable of playing skyrim on max. (You can check those and many other things (among that other computers and reviews) here:
Notebook check)
For desktops I would recommend graphic card along the lines of:
AMD's Radeon HD 7800 since AMD is usually cheaper and less picky in regards to what it fits with (more customizable) and they are quite quite good
For more information about cards (this is for gamers wanting to play games, which I safely assume you are):
Guide to Cards
You can also check this site out for some knowledge about it (keep in mind that the stated card aren't fully up to date):
Guide
Finally keep in mind that the motherboard should fit with the elements you choose, usually if you for instance buy something pre-build or something you put together on a site that allows for one to customize your setup and have them put it together, they make certain that things fit - but keep it in mind. (I tend to spend quite a bit more on the motherboard to get a really good one, as its the motherboard that kinda serves as your base for all other elements, so if your motherboard is outdated you can't find any of the new stuff you want to upgrade your comp with). Additionally the motherboard also decides how many RAM you can fit, amount of USB ports, how good your sound is (quality of integrated soundcard) and so forth.
This also relates to the power supply that needs to be able to power all of it (I personally like to keep some more juice on mine). Also in that relation its a very good idea to have at least a decent cooling system. Makes the computer last longer and if you buy a good one you don't get a lot of noise.
The cabinet is not that important, though if I were you I would make certain to take one with space enough for upgrades.
Finally the hard-drive: you can just buy a harddrive with enough space to suit your needs, I would say around 500gb to 1tb. Since it sucks to run out of space and having free space also helps on your computer speed. In that context you can also consider SSD (solid state drive) which will improve your compute speed and startup time. It is way more expensive, but it also have a lot of benefits. One can also combine the two types, having a normal harddrive (HDD) to store shit on and a smaller SSD for windows and startup.
Alternatively you can use a hybrid drive which is cheaper and carries some of the benefits of the SSD (increased speed among those).
For more on that:
Guide to SSD
Hope this will help you some and good luck with it.
(Mod: if you want to move this to a new thread, your welcome).