I would say a psu is the most important part of any build, and yes you'll need an internal drive to install an OS. Perhaps instead of buying one you could borrow one from someone you know? But there is no point in buying one when your current external ones are working great. Plus drives such as dvd burner etc put very little strain on a psu.
Phenom 9850 x4, Asus MVP Deluxe , 8gigs OCZ ddr2 1066, Visiontek 3870x2, Xfi Elite,2x WD Raptors 10k in raid 0,Antec 1000watt, and Cooler-master Stacker 830 Evolution.