There was little difference in power consumption when running this Zotac card at default clock speeds and underclocked reference speeds, so we've recorded a single figure for them both. We take the reading from a wall socket reader that checks the complete system power.
Clearly, nVidia's hard work to massively reduce idle power consumption for the original 200 series chips hasn't been undone with this latest incarnation. Bearing in mind our earlier observations that the GTX 275 also runs quieter than the HD 4890 when idling, it's a clear choice which way to go if you tend to leave your PC on for long periods of time. While this trend is reversed when the cards are under load, the difference is much less so wouldn't sway our decision either way.
As well as underclocking this Zotac card we also had a go at overclocking it further and had a little bit of success. In fact, we pushed the core clock to 721MHz without too much trouble, making for a total overclock of 88MHz (over default speeds). This works out as a 14 per cent increase, which is near enough the 15 per cent (850MHz to 975MHz) we achieved with the HD 4890.
On the evidence we've gleaned, both the nVidia GeForce GTX 275 and the AMD ATI Radeon HD 4890 hold a clear advantage over what came before and are great value products, whichever you choose. As for which of the two to go for, that's less clear cut.
If you tend to leave your computer on for long periods of time when not gaming, you may appreciate the lower idle power of the GTX 275. Likewise, the wider software support for CUDA, over ATI Stream, may attract you to side with nVidia, especially if you work with video a lot - be it editing or trans/encoding. However, based on the games we've tested, ATI has the faster gaming card overall and for us that's still the most important consideration.