I'll mention a few things; some may be new, some may be redundant.
As for the basic difference in the capabilities between the Intel Pentium 4 CPU (Central Processor Unit) and the AMD Athlon64 CPUs is that (mostly due to the shorter "pipeline" of the AMD CPU and the longer "pipeline" of the Intel P4 CPU) the AMD processor is best suited for Microsoft Office applications and Gaming. The Pentium 4 is a better processor to use if you are involved in Content Creation, 3D rendering or modeling and Video or Audio encoding. BOTH will serve your Internet purposes equally well.
As far as cases are concerned, the ANTEC brand of cases are an excellent Case make by a well-established manufacturer of Cases and Power Supplies. Just keep in mind that these cases (at least at one time) could often be found in nearly identical formats under the name CHIEFTEC.
I agree with YOyoYOhowsDAjello that the way to LEARN what "all this stuff means" is to READ, READ, READ and PRACTICE, PRACTICE, PRACTICE. Reading can be done both on the internet here at ANANDTECH.COM which is an EXCELLENT source of PC news and information as well as the "other" major site that was mentioned called "Tomshardware.com". After a while, via links from these sites, you will begin to disvover websites that are smaller, but sometimes more specialized that may suit your needs like hardocp,hardwareanalysis, hardwarezone,legionhardware, pcstats, vr-zone and overclockers.com just to name a few.
There are also plenty of good printed materials out there to keep you up-to-date on the lastest hardware and software news such as MaximumPC and CPU (Computer Power User), just to name a couple of them.
Your first system build may not turn out as well as you hoped but with practice each successive system build will get better and EASIER (USUALLY, lol).
Lastly, there is NO BETTER PLACE to get help with your problems with system building than RIGHT HERE at ANANDTECH.COM 😀