SPF set up for Gmail-NO CNAME & TXT record on same domain?

I was trying to set up SPF even though it is hotly debated on whether or not it will work as someone is spamming folks using my domain.

My email is through Gmail thanks to Dreamhost’s quick and easy set up. So, I got the SPF record from google, I went into Manage Domains and it will not allow me to add a TXT record.

I assume that since my email goes through mail.mydomain.com that the SPF record should go on that sub domain and not mydomain.com.

Yet, the setup for using GMail with Dreamhost has an MX record and a CNAME record pointing to google. Thus when I try to add the SPF I get the following error:
You already have a CNAME record for this name. You can’t have a CNAME and any other record on the same name.

Any ideas?

UPDATE: Ok, so I am an idiot. After running the SPF wizard at OpenSPF it would seem to indicate that the SPF TXT record does belong on mydomain.com, not the subdomain.

Unfortunately, the SPF record didn’t help as they are still using my domain. I assume since Google is so popular the spammers are using their mail servers so I made an SPF record for nothing.

Setting up SPF won’t stop spammers from sending email using your domain — nothing can. What it does do, though, is make it easier for recipients to detect spam sent using your domain, as long as they’re using a competent spam filter.