Hello!

First of all, let me tell you what I have.
I have a residential broadband ADSL connection with a static IP. Download is 15 Mb/s and upload around 800Kb/s.
My home future server is just a laptop. 1,6 GHz, 2 GB DDR2 RAM, XP Pro SP3. Generally speaking, I use Joomla to build sites. I plan to have a web site hosted at home and also have a SMTP server to manage e-mail.

I have a domain name and I want to make sure that I did everything right with DNS configuration.
The web site that manages DNS has the following fields:

in A record there's a SUBDOMAIN box where I just put @ sign (it means root, right?), then there's a default TTL 7200, and IP ADDRESS when I put my static IP. The next line is CNAME record and in SUBDOMAIN box I put www and then 7200 TTL and then the HOST NAME where I just typed mydomain.com
The next line is A record for a mail server. In SUBDOMAIN box I have smtp, then TTL 7200 and finally IP ADDRESS.
And the last line is MX record. In SUBDOMAIN I have smtp, then TTL 7200 and then PREFERENCE 1 and the last thing is HOST where I have my domain.com
Are those parameters correct?

I plan to use Ubuntu Linux NON Server OS with Apache on it (probably LAMP) and for SMTP Server I'm not sure...I found Citadel. I used to have hMail couple of years ago and I really liked it, but it works on Windows only.
Frankly, I don't like Linux, but I think that in my situation its the best bet. One thing is not clear to me though...Why would someone use a command line Server Ubuntu Linux when there's a GUI version of Ubuntu that runs Apache as well?

You cannot run a server on a 800kbs DSL connection, your "upload" speed is the connection speed for anybody accessing the website.

And you CERTAINLY do not want to be trying to configure a webserver on what is esentially a desktop / workstation version of Linux.

Oh and;

If you are planning to have a live SMTP server running, do you know how to beef up the security on mail servers so you can avoid being hit with spammers using your server to send mail through withing hours of going live.

And the answer to

One thing is not clear to me though...Why would someone use a command line Server Ubuntu Linux when there's a GUI version of Ubuntu that runs Apache as well?

Because it's quicker and easier!

I run Ubuntu Linux on my desktop and run the Apache server package to serve a web site on my local network.

Why is there an Ubuntu Server edition when it's just as easy to run Apache from the Desktop version? Simple. More computer resources are required to put up that pretty user interface. By the time you get php and MySQL and other things running on there, your Desktop will be very slow in processing server demands. The Server Edition devotes more horsepower to running the server's needs and does away with the point and click and graphics.

You say you don't like Linux much. In my book it sure beats Windows XP and its successors, for I never have to worry about virus attacks, never have to check the disk for errors and defragment the drive to keep things running (a check is scheduled once every so-many boots automatically), but I can open the Terminal and type "sudo touch /forcefsck" without quotes and enter my password any time I want to force a disk checkup on my next boot. I never am running needlessly with Administrator privileges, but they are a "sudo" away if I need them, say to install or remove software, change system settings.

Back to the main topic: A household Internet service is set up to let the end user download quickly from many sources, on 12 lanes of a superhighway, but can only upload on 2 lanes going the other way, at a reduced speed limit. The Web host's service is set to do just the opposite. You upload the Web site to the hosting account over an upbound 6-lane road with a reduced speed limit and the server spits it out on a 20-lane downbound superhighway to all of its visitors.

You know, it's the occasional gem such as the above post that keeps me coming here to read.
It is not self-serving in any way, it simply demonstrates a good working knowledge of Linux, hosting and why different versions of operating system exist.

+rep given