I got a server running at home. Mostly its an apache and mythtv box.
I now want to know how i should set it up, that it can work locally over WLAN and extern via 3G?
Should i use a VPN for that? If yes, how can i set this up? I know that i cant connect from the localnet to my server by using the dyndns name, so i cant set this static.
Any ideas welcome.

First off it's possible, using VPN or SSh. Secondly your dyndns url would need to be updated via your router or ISP provider. Most routers today support DDNS which does for you. Once you have that taken care of, your static ip is your dyndns url.

Setting up VPN would require either your router or server to run a vpn server or client (depending on how you want to use vpn). I assume you want to VPN and control your server computer.

Setting this up varies on the software you're using. There have quite a few post talking about, I would just search VPN and go through all of them. Also there should be a VPN wiki somewhere on the web. Check it out, it's pretty universal.

Trust me, it can somewhat of a headache to get running, but once it is, it's amazing.

2d

Dyndns is no problem, its already set up. OpenVPN should be no problem to set up, too, as i did this short time ago.
The Problem is, that that wont work with the dyndns url when you are connected to your local WLAN. The router wont route you out and in again. Thats the only question i am having. How to deal with that?

Ahh, just use a different ip. Instead using the url, use the local ip of the computer since you're in the same network already. For example if the computer's IP is 192.168.0.101 you would use that instead of myip.dyndns.org for example.

if you're saying that the computer is not a static ip, then you can set it static by using a rule in your dhcp setup on the router. In this way the DHCP server give the same IP to the computer without an expiration on lease time, effectivly making the IP permenent and static.

2d

Ah, now i maybe get the idea:
Just entering following line in hosts file in the router may help, or if not configuring the dns in the router to do that mapping. I will look into it. Its an openwrt router so should be doable.
192.168.1.3 mydns.dyndns.com