... why would I consider a 350W or 400W PSU?
The way I look at it, the PSU is like the engine in a car. So, to turn your question around: Why do I need a car that goes 100 mph, if I'm only going to drive it at 60 mph?
Just think of the extra wattage as overhead. There would be less wear and tear on a PSU that has to put out at 25% load, compared to 75% load over time.
If you get a quality 80 plus PSU, that's properly rated, you'll still get a good efficiency rating (80+) for the power you use. You'll also have room down the road if you either upgrade to a better CPU, add a graphics card, etc.
The bottom line then is price. The sweet spot (I feel) for a PSU is at the 400w range. The Corsair CX430 is an 80+ Bronze rated PSU that can generally be has for $20 A/R when on sale.
Now, would I use that same PSU for my desktop, probably not. But I think, for the average HTPC, it's perfect.
Right now, the best bang for buck is probably the SeaSonic SS-300ET I mentioned above (with the discount) if you had to order today.
You can't go wrong with a SeaSonic, Antec, or Enermax PSU.
The OCZ PSU's (I believe) are based off the PC Power and Cooling PSU's.