Verdict: It's no 8800 killer in DirectX 9, but it has lots of potential.

The Radeon HD 2900XT is ATi's much-delayed response to Nvidia's GeForce 8800-series, and a fully-fledged DirectX 10 GPU with a unified shader model.

The HD 2900 XT has 320 scalar stream processors that can work on pixel and vertex data, and also geometry shaders, which are a new feature of DirectX 10. It also has a special programmable tessellator, which enables the GPU to create a high number of polygons with relatively little cost in terms of resources.

Such a massive number of stream processors suggests that the HD 2900XT should destroy even the GeForce 8800 GTX, which has 'only'128. However, the main difference between the two is that the HD 2900XT's stream processors run at the GPU clock speed of 742MHz, which is much slower than the 1.35GHz of the 8800 GTX's stream processors, and 1.2GHz for the 8800 GTS (which has only 96 stream processors).

The HD 2900XT's texture units have also been updated, with improved anisotropic filtering, support for higher-resolution textures than the X1900-series can cope with, and the ability to filter HDR textures up to seven times faster than the X1900-series. Comparing the HD 2900XT's texture units with those of the 8800-series is hard, although its raw filtering performance is poorer (16 pixels per clock for an FP16-format texture, compared to 32 for the 8800 GTX and 24 for the 8800 GTS). That said, the HD 2900XT's higher GPU clock speed should compensate.

In terms of pixel output, the HD 2900XT has only 16 ROPs, compared to 24 for the 8800 GTX and 20 for the 8800 GTS. However, in terms of raw pixel output, the HD 2900XT's high clock speed helps again. With 16 ROPs running at 742MHz, the HD 2900XT clocks in at 11.9 gigapixels/sec, while a 640MB 8800 GTS running at 500MHz churns out 10 gigapixels/sec.

Another part of the ROP's job is anti-aliasing, and ATi has debuted a new Custom Filter AA mode with the HD 2900XT, which it claims provides higher AA quality than that offered by Nvidia's Coverage Sample AA mode.

One area in which the HD 2900XT clearly beats the 8800-series specs is memory bandwidth, thanks to its 512-bit ring bus controller. Add in the HD 2900XT's 828MHz (1,656MHz effective) GDDR3 memory, and there's a titanic 106GB/sec of memory bandwidth, compared to just 64GB/sec for a 640MB 8800 GTS. The only downside is that the card has just 512MB of RAM.

The HD 2900XT is very power-hungry; with the card installed in our test PC, the system drew a mammoth 355W (peak) from the mains, compared to just 269W for the PC with a 640MB 8800 GTS installed. As a result, the fan in the dual-slot cooler has to work extremely hard, making it much noisier than a 640MB 8800 GTS. With such high power demands, the HD 2900XT has two PCI-E power sockets, one 6-pin and one 8-pin, although it also worked with two 6-pin PCI-E plugs connected.

ATi clearly considers the HD 2900XT to be a pure gaming card because it lacks the new Universal Video Decoder (UVD) hardware that will feature in other models in the 2000-series. This fact only came to light after the launch, which is why our original review stated that UVD was supported. ATi has added an HDMI port though (via a DVI-to-HDMI adaptor) that can output an HDCP-encoded video and audio stream. The HD 2900XT can also perform many types of non-rendering calculations, such as physics and folding.

The HD 2900XT was capable of playing Need for Speed: Carbon smoothly at 1,920 x 1,200 with 4x AA and AF enabled, but could only manage F.E.A.R. at 1,680 x 1,050 with 2x AA and 8x AF. This is because the HD 2900XT suffers a big hit with AA enabled, which is a result of ATi optimising the GPU for 'shader-based AA' - a technique that it expects to be more common in future games. S.T.A.L.K.E.R. also proved to be a challenge for the HD 2900XT - it could only play the game smoothly at 1,280 x 1,024.

We also tried overclocking the HD 2900XT, successfully raising the GPU from 742MHz to 858MHz, and the RAM from 828MHz (1,656MHz effective) to 900MHz (1,800MHz effective), which provided a noticeable performance boost.

The Radeon HD 2900XT has many strong points, such as its incredible shader processing power and enormous memory bandwidth, which helps it to deliver amazing performance in shader-heavy games. However, by focusing on shader processing, including shader-based AA, the HD 2900XT takes a huge performance hit when using multisampling AA, and its filtering performance also lags behind that of the 640MB 8800 GTS. In many ways, this masks the true power of the GPU and can mean that it performs worse than a 640MB 8800 GTS in certain games. In addition, the HD 2900XT consumes more power than the 640MB 8800 GTS, runs hotter and is far noisier. We can only hope that its performance in DirectX 10 games is better, although early tests aren't promising.

• Media PC components - 22nd August 2008
• PSUs - 14th July 2008
• Multi-GPU graphics - 20th June 2008
• CPU water cooling - 17th June 2008
• Hard disks - 20th March 2008
• Audio hardware - 29th February 2008
• Graphics cards - 24th January 2008

• Samsung SATA SSD 32GB - Hard disks / storage
• Spore - Games
• Zalman VF1000-LED - Other cooling hardware
• Arctic Cooling Accelero S1 Rev 2 - Other cooling hardware
• Antec Mini P180 - Cases
• Turok - Games
• ATI Radeon HD 4870 X2 - Graphics cards