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It's come to the point where I need more than basic assembly knowledge and I'm looking for some good texts on it. Anybody know of any?

I know you also use a Mac, but I'm assuming you mean x86 assembly (under MacBochs). If this is for Mac programming, I'm afraid really don't know any really good PPC text.

The one I usually suggest is Assembly Language Step by Step (Duntemann, Jeff; John Wiley & Sons; ISBN: 0471375233; 2nd edition, May 24, 2000; http://www.duntemann.com/assembly.htm). However, that's a beginner-to-intermediate level text; also, while it uses NASM, the first half is in MS-DOS, while the second half is in Linux.

Unfortunately, I really don't know many really good intermediate-level x86 assembly texts. For advanced assembly, to be honest, the Intel manuals end up being the best source of information, along with a solid understanding of general programming techniques; The Art of Computer Programmng actually is quite useful, as it discusses assembly programming eclusively (in, admittedly, an abstract assembly language). Aside from that, the Graphics Programming Black Book is an excellent source for assembly programming techniques, both for graphics work and in general; even though many of the specifics discussed are now out of date, it is highly recommended even for those without an assembly background.

Should I go for NASM or GAS? And how easy is it to convert from one syntax to the other? NASM looks promising...

Despite rumours to the contrary there's not that much difference between the two in terms of difficulty (Just don't try switching between NASM and GAS in the same coding session ;D). Converting between them essentially just takes some willpower and a quick read of the appropriate manuals.

Further to Schol-R-Lea's suggestions you might want to take a peak at "PC Assembly Language" by Paul A. Carter. It's a nice introduction and the PDF is freely available on the net (Google is your friend).