Actually, you don't want to get the lowest watt PSU you can find. If your computer uses an average of 150 Watts (continuous), then your PSU will only draw what it needs from AC. If you have a 400 Watt PSU and its AC/DC conversion is 400/150, then you have an incredibly inefficient (and crappy) PSU. Your talking about a 37.5% efficiency for the above numbers. That means your wasting 62.5% of electricity. In my opinion, the minimum acceptable range of efficieny should be no less than 60%. Again, a good PSU will only draw what it needs. A better conversion rate for said PSU could be 400/280 or 214/150 (eff. 70%) So, you can buy 600 Watt monster from Enermax for a budget PC, even though it is a bit overkill.
But I wholeheartedly agree with you about amperage. That's one stat that doesn't get enough attention. Technially, when picking a PSU, you want to make sure all is up to your standards, including amperage on rail lines.