Sorry I can't give you everything your looking for here as I am fairly new to building computers as well but I can tell you what overclocking and SLI/Crossfire is.
Overclocking is basically a thing you do to make certain parts of your computer run faster this website does a fantastic job of explaining it http://www.pcstats.com/articleview.cfm?articleID=1804.
Although making the parts in your computer run faster may sound good there is risks. For example if you do not take care when overclocking you could damage you computer and reduce it's lifespan and also check to make sure the manufacturer of the parts you will get support overclocking as some do not and this will void there warranty.
SLI/Crossfire - They are basically a technology used in certain Nvidia (SLI) and ATI (Crossfire) graphics cards that allow you to put in more than one graphic card to make your system even more powerful. You can not use different cards of the same brand (Nvidia, ATI) in most cases but there is a few exceptions. You can also use the same card but different memory amounts so say 512mb in one graphics and 1GB in the other although the graphics card with the larger memory will lower the amount of memory it use to equal the other card, in this case 512mb
Hopefully I have cleared up what both of these are and I hope you have fun creating you computer