We have a couple of articles on installing iRedMail that might help:

https://www.digitalocean.com/community/articles/how-to-install-iredmail-on-ubuntu-12-10-x64
https://www.digitalocean.com/community/articles/how-to-install-iredmail-on-centos-6-3-x64

But they are both x64, my current droplet is x86. Would it matter?

Can I install the webmail on another droplet and use it with the same domain of the first droplet?

> "But they are both x64, my current droplet is x86. Would it matter?"

I don't think it'll matter. I recommend installing the mail server on a separate droplet.

> "Can I install the webmail on another droplet and use it with the same domain of the first droplet?"

Yes. You'll only need to add MX records to your domain that point to the new droplet.

Can you point me to the MX records tutorial please?

Who is your domain registrar?

Godaddy

Add an A record with the hostname of mail and your droplet's IP address as the value, then create an MX record for @ that points to mail.yourdomain.com:

http://support.godaddy.com/help/article/7924/adding-or-editing-mx-records

Thank you mate.

One last question, is it ok if I installed it on my currently droplet and not a separate one? Will it slow down my site or anything?

You can install it on an existing droplet along. However, if that droplet crashes because of running out of memory, or a kernel panic, or if your mailserver crashes because mysql is taking up too much memory and not leaving enough memory for the mailserver you'll lose emails.

If your current droplet has enough room for a mailserver, you can install it on it instead of a new droplet.