From the refresh of the Power Distribution in PCs article, it appears that a low-end computer needs at most 300 to 350W when running the load tests, and may less in "casual" use (net browsing, music listening, DVD viewing).

I'm planning to upgrade my wife's computer doing the summer, so I'm starting my homework ahead of time
The system is gonna be bassed on a 100W AMD Trinity APU (to be released Q2 2012) and a Radeon 7750 with a 61W TDP, so 350W seems fine, 400W a bit on the high side but still OK.
Seasonic seems to be one (the only) reference, with the X-400 fanless being about the only choice for a lower power, gold-certified, modular PSU available on the market in Europe (and the only ATX form factor PSU recommended by SPCR @ 400W).
What other PSU models would the SPCR community recommend?

Recently, I've been using Superflower 350W 80Plus Gold which is quiet at idle (Global 120mm at 700RPM). Can't say much about its full load as I never stress my media box. Except modular cable/fanless cooling, I think it gives Seasonic X400 a run for its money. I bought it less than half of X400 pricing. Recommend for those pursuing quiet, efficient, relatively cheap low-wattage Gold PSU.

In Britain and most of Europe, there is the 350w Gold model from Be Quiet, available from sources such as this one http://www.scan.co.uk/products/350w-psu-be-quiet-efficient-power-bn185-92-eff-80-plus-gold-quiet-fan-atx-v231. At just under £48 this, in the UK at least, must be one of the cheapest Gold PSUs currently available. The only downside may be that it is not modular, but at that price this may not be so important.

Edit: Be Quiet claim that this is the only Gold 350w presently on the market http://www.be-quiet.net/be-quiet.net/index.php?StoryID=17&ProductID=242&websiteLang=en.

I think the Seasonic X400 is a good choice, as it might allow different options in the future, even a mid end dedicated GPU etc. But if you feel you wont be using much more and want a silent PSU, you could try picoPSU and alikes, they have a lot of sub 200W PSU depending on your needs, for example Premium picoPSU-160-XT/16A 192W AC-DC, this would also allow you for a smaller footprint case as they use an external brick, but it really depends on each needs, i been running mine more more than a year and im pretty happy with it, and under very light loads they consume less than higher watt PSU, but as i said been also pretty happy with my X400, both are good choices depending into what you aiming, as an alternative you can also look into. Another option, probably not as good as the seasonic, but seen decent comments is the FSP Group AURUM GOLD 400W, much cheaper but i haven't tested personally to know first hand, i really wish FPS would give SPCR a sample to see how it stacks with X400.

To some extent, I think BeQuiet's bold claim makes sense due to limited availability of low wattage Gold PSUs in different regions. For example, BeQuiet doesn't sell this 350W Gold baby here despite the fact that I can purchase BeQuiet 550W Gold. To tell the truth, we're the minority of retail market who want to acquire lowest wattage/highest efficiency PSU. Something below 500W is defined as small/low by today's standard...

What's wrong with more (power) capacity? Are you worried about lower efficiency? Higher price?

I've got a 80+ Gold 400W Be Quiet PSU.

I'm mostly concerned about efficiency: what's the point of using a 500W 80+gold PSU, when typical load range (idle/internet surfing) is way below 100W, thus below the 20% load efficiency milestone?

Don't worry about maximum power rating, just get the cheapest Seasonic X series, whether it's the 400, 560, or whatever. They are all fanless at low power.

Cant help but second this suggestion.

Does efficiency really matter if load is so low anyway?

I'd recommend Super Flower Golden King Platinum+ series. 500W or 550W Pro model should be totally silent (fan is not rotating at all).

I think it is identical to Kingwin Lazer Platinum 550W which is reviewed here:

http://www.silentpcreview.com/article1203-page1.html

Don't mind the actual wattage which is more than you need. The important thing is high efficiency (>90% at 65W!) and semi-passive design. I'm gonna get one of those anyway. Seems to be the best bang for a buck if you want silence.