No, a 300W PS is not necessary.
I have a 800 T-bird running at 950mhz, ABit KT7 Mobo, a Plextor 12/10/32 burner, IDE CDROM, Klipsch Promedia sound system, 2 IBM 7200 Harddisks, Sound Blaster Live, GeForce2 MX w/fan @ 210/210 and a Alpha 6035 cooler. All running 100% stable with a 235w PS. Yup, you read correctly - 235w PS!