Hi phenom01, 400 watt is barely enough for a GTS 450 especially if you wanna play 'round with stock settings. I'll definitely look for around 40 amps on the +12V Rail as you have to factor in capacitor aging at least, if not any future upgrades.
One thing I missed in my last post (& sorry for that) is that you're not a serious gamer. Your primary requirement is net surfing/multimedia/home entertainment. If you're only a casual gamer, you should actually save some money by getting a EVGA GeForce GT 430 (Fermi) 1GB
or a XFX Radeon HD 4670 1GB
if your main requirement is net surfing/multimedia/home entertainment.
There's absolutely no need to get such an expensive graphics card, IMO.