"Trivial Question" - 32bit vs. 64bit vs. 32/64bit

[Chris Horn <chris@beefstew.net>]

I currently have a fully functional (and I'd like to keep it that way) SMP x86 
box (Abit BP6); it's been running Debian testing for several years.  Come the 
beginning of next week, I'm going to swap out the motherboard, CPU & RAM for 
a single proc Opteron based system (Asus SK8V).

I have read/skimmed the AMD64 Debian HOWTO, but still don't know the answer to 
the general question: Do I want a "pure" 64 bit system, or not?  It seems 
like there's a 32 bit "compatibility" mode (that had a root exploit), but I 
don't know how that works (generally speaking).  Do I have to keep around a 
whole separate /lib32 directory (as opposed to a /lib64 one)?  How would I go 
about running GL games, like Quake, Critical Mass and Return to Castle 
Wolfenstein?  Does ALSA work in 64 bits?  Do nVidia's video drivers?

Applications aside, I thought the Opteron could run either 32bit stuff or 
64bit stuff natively.  If this is so, then why does there need to be a 
software compatibility mode?

Can someone please help me out with some of the basics of migrating to 64bits?  
I promise I'll try to generate some sort of document to help out others 
(suitable for addition to the HOWTO).

Thanks in advance.
Chris.