Having troubles modifying the loop

Hello all

I’ve been trying to modify my loop. On category and tag pages I’d like to have to posts listed the following:

–> first show all posts for this category that are NOT also listed in category X
–> then show all posts for this category that are also listed in category X

I did some reading and managed to achieve that with query_posts, but this causes problems with the pagination … (and apparently it’s a bad idea to use this method anyway)
I tried WP_Query but then it shows all posts, not just the ones for this category.

Can anyone help me out?
Thanks in advance