does it matter what type of processor the computer has?
Not really. Linux will run on most anything.
To answer your questions.
#1. Its Ok, if a little dated, but that should't matter too much.
#2. This really depends on weather or not you want to use both drives. If you want to dual boot, you may aswell just leave the original one in there and make the new one a slave.
#3. This really depends on what distro your using. There are user freindly distro's (Suse, Fedora, Mandrake) that setup hardware drivers for you. Then there are more involved distros (Gentoo, Slackware), where you'll need to configure the kernel (and drivers) yourself.
#4. In my opinion. LAMP is just a phrase. I dont believe there could really be any detailed LAMP book or guide as each component (Linux, Apache, Mysql, Php-Perl-Python) is a broad topic in itself.
The best thing you could probably do, having not used Linux before, would be to get your hands on a more user frendly distro (I got a copy of Suse free with a magazine) and install it. From there you should be able to find a guide relevent to that particular distro on installing apache, mysql, postgres, php whatever.
Then, well, its just a matter of playing around. I used Fedora for about 8 months before I decided I wanted to try a more hands on distro and moved to Gentoo. It just depends how involved you want to get and how much time you want to devote to it (the learning experience).
Hope this helps, at least some.