Absolute Bottom Line Minimum for what you're running 300WATTS
Safer at 350WATTS
No Problems at 400WATTS
AMD XP requires a Min of 300w,
Gf4 TI-4200 requires a Min of 300w,
Add a couple of cooling fans and you've got trouble, I would reccomend to get a 400W and be done with it, you didn't scrimp on the video card why scrimp on the power supply.
Details, Details, Its all in the Details, If you need help, Don't leave out the Details.