The 400 is to indemnify the company. It should be fine.
I recently installed a Radeon HD 5450 on my desktop and found out that it needs at least 400W. I have a 250W PSU and I'm having trouble finding the right PSU. I want a PSU for at least 30 bucks and not something crazy like 90 bucks. I have a Compaq Presario SR5250NX. If anyone can help that would be appreciated. I'm really noobish at finding the stuff.