Upgrade the power supply on a Sony Vaio Desktop (PCV-rx462ds)

Hello!
I recently bought a better video card (GeForce FX5200, 256MB VRAM) for
my Sony Vairo Desktop (P4 1.3 GHz, 512MB RAM). Unfortunately, my power
supply is inadequate, and the only way to get the card to work was to
disconnect my DVD-ROM from the power supply. I want to get a new power
supply (300Watts or more), but I don't know (a) which ones will be
compatible (there's no label on the current power supply), and (b) the
risks of installing a power supply with higher wattage than the one
the system came with. I would really appreciate responses from anyone
with answers to one or both of these questions. And if there's
anything I'm missing, I'd appreciate being made of aware of that, too.
Many thanks!
TM