newbie questions

[Sebastian Schubert]

Hi,

I'm new to gpg and I'm sure you can help me.

I created two main keys (twice --gen-key) and I added a second user ID 
to the first one. I can choose which one I take with the -u option. But 
how can I choose which user ID of the first main key to take? Do I 
always have to change it with "primary"?
I signed a document with the first key and then I decrypted it and gpg 
gave me both email addresses (I guess you know the English version):

gpg: Unterschrift vom Fr 11 Feb 2005 00:22:37 CET, DSA Schlüssel ID 
130EAA7E
gpg: Korrekte Unterschrift von "Sebastian Schubert <xxxxy at yyx.c>"
gpg:                     alias "Sebastian Schubert <yxyx at xxc.y>"

So does it make sense to use several user IDs when everybody can see 
everything immediately? When I add or delete a user ID, do I have to 
get the signs for my key again?

Thanks for helping me
Sebastian