Where are you based out of? As mentioned, you should try and focus on one carrier to reach the highest possible status ... that being said, with all of your Eastern Seaboard flying, NW may not be your best choice ... I'm a bit biased, but you may want to consider US Airways if they are convenient from your hometown, they certainly have the best coverage over the region you describe next to Delta (whose Elite program is problematic at best at this point).

Of course, if you're in ATL or CVG, you'll probably just want to stick with DL ...