That being said, what else are you powering with your 400w PSU and what is your +12v amperage?
If it is a low end POS PSU, I would highly recommend upgrading to a quality Corsair, PCP&C, Seasonic, Enermax or Antec Earthwatts PSU.
The problem with 98%+ of these no name ultra cheap PSUs is that they way over inflate their capabilities and use very cheap components. The PSU is something you NEVER want to skimp on as when a cheap one fails, there is a high probability that it will surge voltage into your system and kill other components.
I run an MSI R4670 in a Dell Dim E510 Pent D-805, 2 GB RAM, and extra HD on an oem 305W PSU without problems. I play BF2 on med settings at 1024X768 or 800X600 no problem. No meltdown or explosions yet.