Hmmm... what does this remind me of?
Oh, I know!
The Hellboy Roleplaying Game
But I don't blame you for not having heard of it, and I'm not really a huge fan of the GURPS mechanics. (Like many GURPS books, though, the background info alone is more than worth the cover price!)
If you're looking for a better system, ask yourself this question: do you want normal humans, like FBI agents or nazi soldiers, to ever be a threat to the demon PCs? Or should giant chaos gods and undead mad scientists be the only true challenges?
If the former is true, go with
D20 Modern. It already includes rules for Hellboy-ish things like pyrokinesis, telepathy, magic, and demons in a modern world. Better still, it provides a way for more ordinary characters to be somewhat balanced against supernatural characters. The only extra rules you'll need are the stuff for playing monsters as characters that you'll find in your D&D books.
If you want humans to be uniformly weak, go with
Mutants and Masterminds. The system is fun, combat-oriented, and straightforward. But it's balanced for superhero vs. superhero play, not superhero vs. normal play, and non-combat encounters (investigation, persuasion, stealth) are not well-supported.
I hate to diss any system, but Deeds Not Words is one of the few times I've really experienced RPG buyer's remorse. It's got great art and some fun concepts, but the mechanics are a mess.