ATI Radeon HD 4870

We're still enthusing over the ATI Radeon HD 4850, but already its bigger brother has arrived. Can the red team continue to set new standards in graphical bang per buck?

The Radeon HD 4870 is a meatier card than the HD 4850, occupying two slots instead of one and taking two six-pin power connectors instead of one. However, as we reported at the official launch , internally the two are extremely similar: the new design is based on the same RV770 core, and still offers 800 stream processors and 40 texture units. The only difference is a clock speed that's higher by a fifth - 750MHz versus the lesser card's 625MHz.

Launch models of the HD 4870 also ship with the same 512MB complement of onboard memory - though 1GB variants are expected in the near future.

But while the amount of RAM may be the same, ATI has equipped the HD 4870 with cutting-edge GDDR5, rather than the GDDR3 found on Nvidia's cards and the HD 4850. This technological advance means that, even though the 900MHz RAM clock isn't especially high, effective memory bandwidth exceeds 115GB/s - almost double the 4850's 66GB/s.

Indeed, the HD 4870 can in theory throw textures around internally faster than any other card on the market bar the top-end GTX 280, which sells for almost twice the price.

Real-world performance
The real question, though, is how the upgraded clock speed and RAM technology affect real-world performance. Our benchmarks provide the answer.

In our medium-detail Crysis benchmark, the HD 4870 averaged 63fps - a score identical to the HD 4850's. Turning up the details to high, however, set clear blue water between the two cards: the 4850's 32fps was playable, but the 4870 raised that figure to 39fps.

To put these scores in perspective, Nvidia's GeForce 9800 GTX, which sells for a similar price to the HD 4870, stalled at 36fps, and even the GeForce GTX 260 (now selling for around £230 exc VAT) only equalled the 4870's high detail score.

The Call of Juarez benchmark set the HD 4870 in an even better light. Its high-detail average of 40fps was the highest we've seen, beating even Nvidia's GTX 280, which scored 37fps. And it's nearly a 30% improvement over the HD 4850's score of 31fps, indicating that the switch to GDDR5 does boost performance beyond what you'd expect from a clock speed increase alone.

Performance vs price
For £161 exc VAT, these are remarkable scores, and next to Nvidia's current offerings the HD 4870 looks like an incredible bargain.

Unfortunately, next to its own cheaper sibling, it looks slightly redundant. Sure, performance is 20-30% ahead of the HD 4850, but the price is some 60% higher. Since the cheaper card happily handles Crysis at 1,600 x 1,200 with high detail, it's hard to justify paying that premium unless you're a real framerate junkie.

Still, the HD 4870 is further proof of the power of the RV770 GPU, and as prices fall it's just going to look better and better. If you're seeking more juice than the HD 4850 can deliver, the HD 4870 should be your first port of call.

Author: Darien Graham-Smith