Yeah. That 300 watt might just do it though it would be a push. I guess it depends what else you run on the system.
Anyway, if you notice any problems, you can easily buy a 400 or 500 watt PSU and replace the old one can't you?
If anyone out there is using AGP though, it is now a rule of thumb that the best bite for your buck is XFX GeForce 6600 GT Video Card, only $175 some places. It is not the best overall, but it is the best for slightly under $200.