Feeding the voltage back into the supply which what you are doing when applying the 7V trick is fine for a few fans. For a pump probably not a good idea.
The Watt count especially on generic power supplies don't count it's the Amperage. Also generics tend to lie or otherwise hide how much they can really output. A quality 300W could easily handle the system being a P4 era Dell.
[SOLD] 775 Motherboards
SOLD
For sale are two solid ATI Crossfire capable feature full highly overclockable 775 / DDR2 motherboards.
These Items were stored in a smoke and pet free environment, are in mint condition, and are tested fully working.
Let me know if you need any specific...