Being as the HD 4850 uses the same chip as the HD 4870, you'd expect it to have roughly the same power consumption figures. However, the change in clock speeds and memory type obviously makes quite a difference as this is the most power efficient card on test when idle and only draws more power than the significantly slower nVidida 9600 GT under load.
So, if you want a relatively small single slot, power efficient, yet fast graphics card the ATI HD 4850 would certainly seem to fit the bill perfectly. If you then want an extra performance boost, going for an overclocked card certainly seems to pay dividends as well.
We're still rather concerned with this card's heat output, though - to the extent that we'd hesitate to recommend it for small form factor cases like Shuttle PCs, which would normally be a natural environment for a single slot card. That said, for more well ventilated cases the heat shouldn't pose a problem.
One final problem for the HD 4850 is actually the HD 4870. With both nVidia and ATI fighting a fierce price war around the £200 mark, the HD 4850 has been left behind a bit and is now only £50 cheaper than the HD 4870. While that's still a fair chunk of change, if your motivation for buying the HD 4850 is purely financial (as opposed to wanting the single slot form factor, or lower power consumption) then saving for a few more weeks for the HD 4870 would be a sensible bet because the boost in performance is significant.
The ATI HD 4850 doesn't quite have the stunning price/performance ratio that earned the HD 4870 an Editors Choice award. However, it is still a very attractive option for those that simply can't push there budget further. Performance and power consumption are class leading and on every other level its on a par with the competition so all in all it gets a solid Recommended.