Apache is by far the most common server on the net, and a very good one at that.
If I were you, I'd download the source and compile it, mostly cause some Linux distributions tend to place config files in different locations, while with a compiled version, you'll get them in the "standard" localtions.
And unless you're looking at a very high traffic website, you'll likely be fine with any P5 class CPU or better, though a P6 would be preferable if you're gonna run anything else on it as well.
Perl is a programming language, quite useful for a bunch of things, including CGI scripts, you can find info about it all over the net, just use google.
O'Reilly have some really good books about it as well.
And as for doing other things as well, you can start processes with different priorities, using "nice", just look at the manpage for more info.
Or to change the priority of an already running program, run "renice", again look at the manpage.
A few useful sites:
Linuxnewbie.org
Linuxdoc.org
Perl.org
O'Reilly.com(for good books about just about anything Linux related, including Linux, Perl, and Apache).