Building my first (gaming) computer, what do I need to know?

There are lots of resources online that you can go to in order to learn more about building computers, http://www.logicalincrements.com/ is one of my favorite sites because it's really easy to use and tells you what everything is doing in your computer and what to look for (scroll down a bit past the recommended builds). Looking at old threads on sites likes tomshardware is also a great way to learn because there is a massive amount of knowledge that is being shared.
Learning what you would consider overkill is all subjective, and all depends on what you think you want in the computer you are building, for instance if you don't plan on playing games then it wouldn't be a very good idea to invest a large amount of money in a graphics card. Its good to come up with a specific question, such as "how much ram do I need" and to google it, because I guarantee you there has been another person that's asked the same question on some forum somewhere, and so there's lots of literature out there.
As for monitors? My rule of thumb is that it has to be under 5ms response time, which says that the image on the screen is being delayed at most by 5ms. Another spec is the refresh rate, which can vary a lot, but for a smooth image you want to get a 60hz monitor at the least. I like to try to get a 1080p monitor, but a 720p one would do just fine if you're on a budget, because every game I can think of will support it. The size is just however big you want to go, I like a 24 inch monitor, but that's me.
I tend to tell people that if you need to know something, then just google it.
Let me know if you have any other questions or if I missed anything.

Edit: I've found the best picks on tomshardware as a very helpful guide, but bear in mind that prices change and you may be able to find something better than what they have if there happens to be some deal going on, which happens all the time for various reasons. I like to buy my parts from Amazon, Newegg, or microcenter, as they tend to have very good selections for a good price. Customer service doesn't really matter as you will be dealing with the manufacturer directly.
I've put together nearly a dozen builds for friends and family and myself over the past 4 years, and I would still consider myself a novice.