Lol thats weird, Reghakr and I have just been chatting about the same thing..
I totally agree about modular, I use a modular system and it's fantastic as you plug in the cables you actually need.
If your gaming on a single card then anything from 500 to 650W should be ok. Dual cards and above need anything from 800 to 1000W..
So what type of system are you going for? Single or dual card?