The IA32 manuals from Intel are quite useful as well: IA-32 Intel® Architecture Software Developer's Manual, Volumes 1, 2A, 2B, 3A and 3B.

Alright, I'll start checking in to the stuff mentioned. Now how do different archetectures affect how I write my code? Or are we just talking about processor specific operations?

You should be sure of your target architecture before you choose the language. Although converting between different dialects of similar architectures often involves no more than a 'find & replace', more dramatic conversions can be the bain of a coder's life.

If you are developing a cross-platform program, you should avoid writing in assembly language. Platform independence is one of the main reasons C (and C++) was invented. However, if you're just setting out to learn ASM to expand your experience of low-level computing, then it really doesn't matter: Once you know ASM, coding for a new architecture from scratch is usually trivial.

My advice is to start off in common ground, using Intel x86, then when you need to go deeper, use whatever your processor likes.

Regards
Admiral