Help with DNS record configuration

[#1]

I have a domain name that I want to point to an alternative server. I've been given a name to point to with a CNAME record
I really really DON'T want to disturb email though.
can I just add an entry for "www", my curent site includes a rewrite rule to add the www so if anyone types it without the www then the rule should correct it and the corrected URL should pick up the new site?
Or do I select "I want to specify DNS records for hosts under this domain"?

[#2]

Quote from: oliverb

Or do I select "I want to specify DNS records for hosts under this domain"?

That's the one you want, just make sure you leave the Email settings section alone.

[#3]

Well that's where I got confused, and actually I ended up breaking things for about half a day by selecting that but then neglecting to add my own DNS records.
It looks as if just adding a DNS entry for the "www" prefix worked. "www.coaxialpower.com" now points to the new host, and "coaxialpower.com" without the "www" still points to the plusnet server, which is set to redirect it. This is working well for me.
Initially I had selected "I want to specify DNS records for hosts under this domain", but this broke the non "www" form of the address because I don't know how to write a DNS entry for the domain itself without a prefix. I'm guessing that on the left I'd put "coaxialpower.com." with a trailing fullstop because on the help page there's something greyed out there that looks like it might be a full domain, but its not clear. Or would I just leave the left side blank?
The next thing I'd like to try is pointing "www.rfpower.co.uk" to ccgi. I seem to recall that involves filing a ticket to get something changed? I have some housekeeping to do first.