[CentOS] yum repository conflict resolutions

Hello,
    I'm setting up a test mailserver on centos4. I want to use stable rpm 
packages but ones that don't have conflicting lib requirements. I've got the 
standard yum configuration with one excdeption i've enabled the centosplus 
yum repository. I've read that dag's repository has a package i'm going to 
need courier so i figured to set that up. Here's my problem, if i then do a 
yum update on this box and more than one repository has a package say kernel 
which does it get? Or, if a package has a library dependency and that is 
installed and it conflicts with another, how would this be resolved? Sorry 
about the confusion any help appreciated.
    I'm also going to possibly have a need to roll out custom-made rpms not 
sure yet on that one, and it's been a while since i did them, rh7 i think 
was the last time. Does anyone have a Centos-specific howto on this not for 
modifying an existing srpm, but for creating an rpm from tarball?
Thanks.
Dave.