Power Wattage

I recently discovered that my branded computer has only 250 watts as its power supply. I wanted to know if this may be insufficient. This is my current computer configuration:
P4 2.4
512 rd-ram
2 – 120 gb hdd’s (the computer came with 1, and I added the second one)
DVD-Rom
CD-RW
128 mb nVidia ti 4600 graphics card
santa cruz dsp sound card
NIC card
floppy drive
4 usb 2.0 ports (no firewire)

If 250 watts is insufficient, then how can I support my claim, i.e. where can I find information that says that 250 watts is insufficient?

Thanks a lot!