Nocturnus-The Key
Samael-Passage, Ceremony of Opposites
i've never listened to syl so im not sure if it sounds like them or not, but its definitely spacey
Alchemist - all of their albums are very spacey but Organasm in particular. Check out the song "Tide in, Mind Out" I think it's called. Spacey as shit.
It'd be cool if there was a sub-genre dubbed Space Metal. I'd like to call it that, but I guess there isn't enough bands aiming specifically for that sound or something.