AMD ATI Radeon HD 3870 X2 dual-GPU graphics cardThe answer i

If – and it’s a big if – you’re thinking of using a pair of Radeon HD3870 cards in CrossFire then the X2 is a better prospect and costs thesame amount of money. The problem is that CrossFire can be erraticwhereas an Nvidia GeForce 8800 GTX is a cast-iron certainty fordelivering the gaming goodies.

Everything about the Sapphire ATI Radeon HD 3870 X2 graphics card isbig, including the model name, so we’re going to stick to calling itthe X2 here.
The principle behind the X2 is simple enough. AMD's ATI Radeon HD3870 is a decent graphics card but it's having a job competing withNvidia's GeForce 8800 family, which is the reason why it was priced atjust £150. So how do you increase the performance of the HD 3870?That's easy - you gang up two of them in CrossFire. But what if youdon’t want to stuff your PC case with two double-slot graphics cards?
The answer is CrossFire on a single graphics card, and here’s the AMD recipe for success. Take two HD 3870 chips - reviewed here- and two batches of 512MB of GDDR 3 memory. Add a PCI Express 1.1bridge chip, mix the whole lot together on a 263mm-long graphics cardand - bingo - you’ve got the X2.
You won’t be surprised to learn that the specification of the X2bears an uncanny resemblance to a pair of HD 3870s while the appearanceis very much like a 3870 with an extra couple of inches tacked on thelength. While Sapphire’s first take on the X2 is a reference design,Asus has done something that is most unusual by producing an X2 withfour DVI ports.
The X2's GPUs are made using the same 55nm production process as theHD 3870, and they support DirectX 10.1 and Shader Model 4.1, and have320 Stream Processors in each chip. That’s 640 Stream Processors intotal with a transistor count that has climbed past 1.3 billion. EachGPU has 512MB of GDDR 3 memory to itself, connected to its own 256-bitcontroller, so there’s 1GB in total.
The card is longer than a regular HD 3870 board, but it looks verysimilar and has the same beefy heatsink along the full length of thecard, with a dust-buster fan at the far end that draws cooling air frominside your PC case. The air is blown through the heatsink and exhauststhrough the vented bracket to the outside world.
The core speed of the X2 has been increased from the HD 3870's775MHz to 825MHz, while the memory speed has decreased from 2250MHz to1800MHz. The memory controller supports both GDDR 3 and GDDR 4 so itwould be no surprise if a graphics card manufacturer was to come upwith an X2 with 2GB of fast memory. Whether it would make any sense isa completely different question