I was just wondering if it is illegal to run a server off a residential cox line. Anyone know?

The Cox AUP says:
Servers. You may not operate, or allow others to operate, servers of any type or any other device, equipment, and/or software providing server-like functionality in connection with the Service, unless expressly authorized by Cox.

But that's pretty much bullshit. Anything that opens a listening socket is technically a "server."

What kind of gaming server are you talking about? Like hosting a network game? That would definitely be fine.
There's really no way to tell if you're "running a server" or not if it's on an obscure port somewhere. As long as you're not running a massive anonymous ftp server or web server (which they block anyway) I'm sure you'll be fine.

I had a CS server running out of my basement for a while, I may put it back up. But I was just wondering about it.

I'm a little out of touch with the newest Cox cable packages. Does their fastest package offer enough upload speed to make this worth it?

Way back when, the upload speeds were not enough to entertain a worthwhile server; you would want at least a 20 slot server to make it worth your time.