You have loads of options here. Probably too many for me to list.
If buying new the most sensible option on a budget is defintely the sempron route. £45 for a chip and £40 for a mobo with integrated gfx. (incidentally the nvidia 6100 boards are good - but the atis are potentially even lower power)
Most semprons will (going by 3000+ venice ratings in tcasemax) probably use between 32W and 44W at stock volts. From there you should be able to undervolt the chip significantly.
If the motherboard allows (or you can use Crystal CPU-ID to do it in windows) it you will probably be able to undervolt the chip as low as 1.1v at stock speed (saving nearly 40%)
Some boards allow you to go down as low as 0.8v cutting the power to roughly 1/3rd of it's original amount ...... however you would also have to start underclocking the chip dramatically too - which reduces power even further!
In theory a 32W venice running at 600mhz and 0.8v would use approx 3.5W full load! :O The sempron would be similar.
If you want to do it even cheaper and processing power isn't an issue there is also the option of using xp-m chips and undervolting (abit boards tend to go down to 1.1v on socket a). Then there are via epia chips or amd geode. Some of the p3s are very low power too.
Personally - for the £85 that a sempron + mobo will set you back and the extra processing power etc you have it's the one i'd go for.
Drop me an email if you want further advice on a specific chip range - I've run low power machines using p3s, xp-ms, bartons, semprons and socket 939 chips. I even wrote my dissertation on the subject
(The 754 and 939s are my favourite partly because they are initially low pwower and can run on such low voltage but also because you can get some awesome cpu coolers for them)
