Radeon HD 4890 XT and OC - cranking up the core
Crankin' up the coreATI's managed to hit an 850MHz core speed on the same 55nm process by going back to the (technical) drawing board and undertaking a critical-path analysis, whereby the RV770 GPU's been extensively screened to determine where the design speed-fails. According to ATI, the time since RV770's launch has given its engineers the opportunity of removing many of the speed-related bottlenecks in the GPU, opening up the possibility of raising the core speed as a result. That's why the regular HD 4890 XT's core/shader is able to run 100MHz (13.3 per cent) higher.
We're not entirely convinced that the main contributory factor here is the reworking, simply because the overall board power, under load, has gone up from 160W to 190W, intimating a healthy bump in voltage. It's pretty safe to say that the combination of voltage increase and core redesign is enough to guarantee RV790s to 850MHz. ATI's further informed HEXUS that the HD 4870 and HD 4890 ASICs are not the same, and we don't expect ATI to move the new ASIC design down to future HD 4870s. That's a shame because the idle-power figure of 60W, down from 90W, looks so appealing.
Overclocking the overclock
Further solidifying ATI's flagship single-GPU card will be the limited-edition Radeon HD 4890 Overclock. RV790 GPUs are screened to determine the very best of the batch and then sold at a slight premium to partners. Rated to 900MHz core/shader, we expect the likes of Sapphire and XFX to use them in even-higher-clocked models - perhaps even 1,000MHz core as standard. Trouble is, ATI's not directly specifying faster memory speeds, so many HD 4890 OCs will ship at 900MHz/3,900MHz.
The long and short of it
Think of Radeon HD 4890 as incumbent HD 4870 with a bit more oomph. Faster core and memory speeds should give the standard-clocked version - 850MHz/3,900MHz - a ~10 per cent lead over an HD 4870 1,024MB, whilst the Overclocked model will add a few extra per cent on top.
The downside to ATI's work is that, at best guess, the HD 4890 XT will cost at least £199, including VAT, at launch, with the OC another £20-£25 on top. Compare that with the £165 for a stock-clocked HD 4870 1,024MB and the new GPUs begin to look just a tad pricey. Yes, they will fall with time, but there's another reason to be worried, and that's the NVIDIA GeForce GTX 275.
