I think they take place in the same world in different nations/continents or at different points in history. I think I read somewhere that there's supposed to be a bloodline between several characters from different games.
First of all what does "seima no kouseki" mean? ive heard nothing about (or read) another FE game for the GBA. Where do u find this info? Do u order some kind of "Future Games Magazines That Tell You Everything Thats Going To Come Out".
Yeah, it's called the Nintendo of Japan website. They have a habit of getting games way before they're announced over here. A lot of other websites, including English ones, tend to cover these announcements as well. Nintendojo.com is a good one as they tend to follow the Japanese press as well, where a lot of these games are announced... though this Fire Emblem has been announced for AGES.
From what I can tell, Seika no Kouseki means roughly: Evil Saint Kouseki (where Kouseki is a name, though rather than evil, it could be "demon saint").