I'm already running a 400 Watt PSU, but I have a few friends in my gaming org that are running 250's and 300's with the GF4 Ti series just fine.
The benefits of a 400 over a 300?
Well for one thing, the extra overhead for things like extra hard drives, more memory and more expansion slots that you can fill without going over-limit on the PSU. If the power supply is of decent or high quality, you can monitor the fans from a Motherboard Monitor software unit, amongst other things. If you use a good quality PSU, or one approved by AMD/Intel (depending on the platform) you should be ok with 300 watts. There might be motherboard incompatibilities to watch out for at that wattage, but it's not something I'd be too worried about unless you're having definite problems.