I think this is probably in the wrong forum.. you should probably move it to the LInux forum or somesuch.. but seeing as there are some hardware questions..
yes, yes and yes - 1GB is more than I have in any of the commercial webservers I run. 20-30GB will be *plenty* unless you're going to be serving some serious quantities of pr0n
and, well the screen doesn't matter - you don't need that .. you can run a headless webserver and administer it from your winXP laptop over the network without too much hastle. I guess all you need is a basic VGA monitor just to get the box installed. you could pick that up for 15 quid in the market
as for server software, Redhat does offer an out-of-the-box firewall option (you'll come accross it during the install). It's basically just a pre-baked list of iptables rule chains. If you understand packet filtering, you could just ignore this and do it yourself. Also, if you're running this box behind an ADSL connection, or similar (and it sounds like you probably are), then your modem/router will most probably come with a built in firewall/NAT/port forwarding collection, so you'll be pretty good from the beginning.
As for the webserver, all you need is Apache to deliver web pages. PHP is useful if you want some server side scripting capability, or if you want to talk to a database.. and yeah, Mysql is a useful enough database to wedge in behind the PHP.
What more could you need? go on then.. get cracking