1. Yes, the specifications of your server will handle your current load fine, and would actually handle much more load.
2. Mandrake isn't really a good server platform choice. As much as I hate to say it, I would be looking into RedHat as it is more thoroughly supported. If you're looking to learn command line and true Unix, you might want to use FreeBSD. If you ever get lost, you can use Webmin [see below].
3. You may need to run SSH if you are going to use remote command line. If not, you might want to look into VNC [ http://www.realvnc.com
] which will allow you to see the machine's GUI over a remote connection. Its default port is 5900.
Another good choice would be Webmin [ http://www.webmin.com
] for remote administration. Its default port is 10000.
4. The best way to learn would be to load up a local copy of the same OS and play with httpd, mysqld, etc. on the machine. That way, if you make a mistake; it won't take down your production web server.
Also, once you get into Linux; buy a book called "Linux Server Hacks", published by O'Reilly. It has a lot of neat tweaks/etc.
EDITED: I made it sound like your server specifications would just not support anything above your current load, which is incorrect.