Don't host a CTF server off your box. It's not worth it. You won't be able to divert enough bandwidth to each player. For CTF run effectively you need to set bandwidth to 7000-10000 bps per player. So, say your connection is a low end dsl, say 512/128. you get 128K upload which actually works out to about 16 kps, so you could have 2 players at 8000 bps. Unless you're on a connection that is atleast 60K (600kps) up, it's not worth the hassle. If the server fills the pings will go to crap. And I'm sure people here are gonna say, well just set it to 4000, because apparently some people are running servers with this low bandwidth setting, but most of these are duel servers. CTF you need alot of players, and the more players the easier the bandwidth get's stretched, hence the higher upstream you need. I rent a 14 person ctf server from gaminghost.com. It's set to 10000 bps per client. So that's a minimum of 140K (or 1400kps). So unless you have a SUPER fast connection, it's not worth it to host a CTF server. A small 4-6 person duel server you might be able to get away with a 128-256K upload speed.
I suggest you join and play on some of the CTF servers that are already up and running, and get used to the gameplay before you even consider hosting one.