The early 04 like mine has no catalytic converter, gets better mileage as noted earlier in this thread. If you aren't going to run egts very high, you shouldn't have any valve seat problems--if you are going crazy with it, get a later head. The earlier trucks have the NV5600 transmission, later ones the aluminum case G56. The overdrive ratio is better with the NV.
You are where I was 3 years ago. I knew I would have to start spending big $ to continue the hp chase on my 97, and it wouldn't do anymore what I wanted a truck for in the first place. I had learned what I wanted taking it to 800 with 1 turbo. AT that time twins was the rage, but now a lot of folks have gone back to a single--with the much better singles that are available now.
You don't have to entirely give up on power with a 3rd gen either. Top dog at MM06 was Reb B wtih 893 on nitrous, using an 03 standard output Cal truck (stock 235 hp). After all the fanfare, I see that the Fall Brawl top number was 101 hp lower than that, with a second gen Ram.