We'll start at the end:
No, you will not have to get rid of Outlook Express when/if you install Outlook, it will be unaffected, and no, we don't need to know about RAM on the machines involved.
Now for the bad news:
You can only use the Outlook out of office facility if your mail server is running Microsoft Exchange, and yours isn't. I've taken a look at your website, and I'm guessing that your email is handled by the company that's hosting your site - the same one that designed it, is that the case?
Outlook is part of Microsoft Office, but you can buy it as a standalone product. There may be other solutions to your problem, but I can't really advise properly until I have a little more information - why exactly do you need the out of office facility?