DNS/Mail Newbie needs help with setting up DNS

Thanks for getting back to me. I'll follow your advice to the letter, but what does the MX 10 bit mean (i mean the '10')? Is that the TTL?
ditto /24 PTR?

The 10 is a preference assignment used in the event that you have multiple mail servers.  It doesn't have to be 10 it can be any number between 0 and 65535 but 10 is the most common (and then 20 for the next MX server record).  

the /24 PTR means that a reverse lookup (eg. a lookup on the IP for that /24 subnet (aka 255.255.255.0)) will correspond to xyz.com

so If I did a 'nslookup your.ip.addre.ss' it would return name = xyz.com.

TTL is generally set elsewhere in the file/configuration.

MX records have a preference value.  The lower valued MX records are attempted before trying a higher number value.  

If you had:

xyz.com    MX 10 myprimarymail.xyz.com
xyz.com    MX 20 mybackupmail.xyz.com

A mail server attempting to deliver mail to foo@xyz.com would attempt to deliver to myprimarymail.xyz.com and if that failed to work, it would attempt to deliver to mybackupmail.xyz.com.

That PTR record might need to be more exact.  If you only have a single IP you might want to do something like (unless you have a full class C address space):

192.158.2.100 PTR xyz.com

Here's a good doc on DNS if you're curious:
http://www.zytrax.com/books/dns/ch8/

You will likely have to ask Rackspace to handle the pointer (PTR) records for you.  Unless they delegate the address space to you, you will not be able to manage the reverse lookup zone.  You can verify ownership of the address space using a WHOIS lookup.  www.arin.net has one on their home page.

I did make a Reverse lookup record with the control panel, that would be that would it?

It probably would be, yes.  In that case leave the PTR out.  You can verify it by doing: nslookup IP.YouSet.In.Reversedns from your computer and see if it returns the hostname that you set for the reverse lookup record.