So i will get in May my first capture card, the Elgato Game Capture HD, but how can i record the game footage in a Lagarith AVI?
Or must i re-encode this?

Elgato records in its own software and uses different codec

And must i then re-encode it?

you will have to encode it with megui anyway, but for an editing program you will need to use settings that won't lose any quality from the recording, plus you will need to record in the highest bitrate that the elgato software can do with whichever elgato you are using

Spazbo4, can you describe me which resolution must i use for wii and should i use an hdmi converter for that?

the wii only has 1 resolution, just make sure its progressive scan when recording wii games, and never use converters for longplays, get component cables for your wii and record with a component capture card, if the elgato you bought cannot use component for recording then don't use it for wii recordings

i had no component cable, i had an PAL wii and only a av cable

then your longplays will be declined using the AV capbles that comes with the wii

ok, then must i wait to holidays then i will get the component cable and the adapter, but for Wii U and switch should the Elgato enough

it depends on which elgato you have, the original elgato is good enough for wii u since that console is good for 720p, but for switch you need an elgato that can record at least 1080p at 60fps, the first elgato is compatible with component cables but any elgato that can do 1080p60fps does not have support for anything other than hdmi, honestly i would recomend the avermedia live gamer extreme since it can do component at 480p for wii output and up to 1080p60fps for switch gameplay but you would also need a pc that can handle it too

i will only get the Elgato Game Capture HD and that can in 1080p only record in 30fps as far as I know

Then you cant use that for any switch game that runs in 1080p at 60fps