Thanks for the suggestion, but again, I have no PCI express just a regular PCI.Put that PSU in and get the PCI low profile 6450 on newegg. The card uses a max of 25 watts. Your P4 will probably use around 120? You're fine.
I wouldn't call the radeon hd 5450 a discrete card, it doesn't require a fan, and under max load it only consumes 19 watts. The card can play 1080p video, barely, and does ok with games, buy I wouldn't call it discrete...