How much wattage do i need?

I am trying to upgrade a dell system with a new videocard, extra hard drive, and more memory.

The current power supply is only 305W and i was worried a larger video card (plus the other things) would require a larger PSU. I tried finding the specifications for my current PSU so that i could replace it however the online manuals weren't helpful. I tried to contact dell and through a live chat i was told that my current PSU would be sufficient.

I dont exactly trust a dell representative (call it instinct) so i wanted to know if you guys thought differently.

I will end up with
2 or 4g of ram,
two hard drives,
2 optical drives,
a Pentium D processor that my computer claims is 3.0Ghz,
and a videocard similar to this

Is 305 enough?

Extra info: Dimension E510