Where's my web server bottleneck?

[bronto]

I have a web server running YDL 2.1.  Performance is acceptable for 
development purposes, but soon one of my sites will start getting 
some heavier traffic.  Already I get an occasional  complaint, mostly 
from visitors who have really high speed connections.  I know that 
this isn't a terribly brawny configuration, but it's been great until 
now, and am prepared to upgrade whatever makes sense.  I have 
suspicions about what needs to be done, want a more thorough 
understanding of what the issues are and what will be the *next* 
bottleneck once I take action.  Here's my specs:

YDL 2.1
Beige G3 tower 233
768 MB RAM (which I believe it the capacity)
Data is stored on 60 gig 7200 RPM IBM IDE drive
Most of Linux is stored on the original 4gig IDE drive
MacOS booted from an old 500 (or so) Mb SCSI drive
original equipment network adaptor (presumably 10 megabit)
connected to internet via bronze DSL (which means 256k up, 768 down) :')

Web applications are Apache, MySQL, Postfix, Courier-imap, and a 
PostNuke application (php).  The php application is at the moment 
uncache because of a lack of cache's for the PPC architecture.  I'm 
aware of some for PPC, but not the ones I want (zend or php 
accelerator).

It's the connection, right?  But when I move it to a T1, where's the 
next bottleneck, and how soon will I hit it?

FWIW, the latest complaint I got was that the main page (about 100k 
in size) took 15 sec. to load.

TIA

Rob