If you mean Dell wouldn't charge $400 for a 250w PSU, clearly they are, but the price of it has no meaning on how good it is. Here's a link for a 400w PSU on ebay that's less than $20 with shipping, it's a Hercules PSU, which is the same kind I have and it works great. This way you can be assured that your computer will run great: HERCULES 400W AMD P4 ATX POWER SUPPLY 400 WATT FAN - eBay (item 270207689817 end time Feb-04-08 20:08:41 PST)
and here are links to Hard Drives: hard drive, Hard Drives - Internal, Drives for Laptops items on eBay.com

If you're harddrive is 510mb I assume it's very old and will likely fail soon but ultimately it is your computer and completely your decision as to what you want

But I will still have 15 watts to spare. And I am not expanding my PC.

If you got that number off the Extreme Power Supply Calculator or any other, keep in mind that it is just an estimate, it has been stated that 100w+ more than the figure they give you is recommended but again I say it is your computer and your choice...I'm just tossing a few options out there. Your parts are compatible though...I did read in your other thread that your Power Supply Unit's fan wires were cut and you taped them together with scotch tape, bad idea, you should use electrical tape.

I wonder what happen when you use scotch?

Scotch tape burns a lot easier and will not stick as long, once it falls off the neg & pos wires will likely touch and short out the system...

I'll use the electric tape then which I was.