Take two, babyOur launch-day pricing comparison showed that a number of ATI's partners had stock of Radeon HD 4770s at various etailers. That wasn't surprising as the initial batch of cards were all based on one design, and partners differentiated on bundle and warranty.
Now, a week later, the stock situation has improved to the extent that it's possible to pick up two identical cards for under £160, combined. That puts the multi-GPU solution firmly in the midst of some heavyweight company, including Radeon HD 4870, HD 4890, GeForce GTX 260, and GeForce GTX 275. Let's bring out the table to see where the two-card solution stacks up. The numbers quoted for the HD 4770 are for a pair of cards.
GeForce GTX 285
|NVIDIA GeForce GTX 275 896MB||NVIDIA GeForce GTX 260 896MB||ATI Radeon HD 4890 OC 1,024MB||ATI Radeon HD 4890 XT 1,024MB||ATI Radeon HD 4870 512MB/1,024MB||ATI Radeon HD 4770 CrossFire 512MB (pair)|
|Memory clock (effective)||2,484MHz||2,268MHz||1,998MHz||3,900MHz*||3,900MHz||3,600MHz||3,200MHz|
|Memory interface and size||512-bit, 1,024MB, GDDR3||448-bit, 896MB, GDDR3||448-bit, 896MB, GDDR3||256-bit, 1,024MB, GDDR5||256-bit, 1,024MB, GDDR5||256-bit, 512MB/1,024MB, GDDR5||256-bit, 1,024MB, GDDR5|
|DirectX/ Shader Model||DX10, 4.0||DX10, 4.0||DX10, 4.0||DX10.1, 4.1||DX10.1, 4.1||DX10.1, 4.1||DX10.1, 4.1|
|Vertex, fragment, geometry shading (shared)||240 FP32 scalar ALUs, MADD dual-issue + MUL (unified)||240 FP32 scalar ALUs, MADD dual-issue + MUL (unified)||216 FP32 scalar ALUs, MADD dual-issue + MUL (unified)||800 FP32 scalar ALUs, MADD dual-issue (unified)||800 FP32 scalar ALUs, MADD dual-issue (unified)||800 FP32 scalar ALUs, MADD dual-issue (unified)||1,280 FP32 scalar ALUs, MADD dual-issue (unified)|
|Multi-GPU||SLI - three-board||SLI - three-board||SLI - three-board||CrossFire - four-board||CrossFire - four-board||CrossFire - four-board||CrossFire - four-board|
|Hardware-assisted video-decoding engine||NVIDIA's PureVideo HD - full H.264 decode and partial VC-1 decode, plus dual-stream decode||AMD UVD 2 - full H.264 and VC-1 decode, plus dual-stream decode|
|Current etail price||£275||£199+||£150+||£210||£199||£140/£160||£160 (pair)|
Taken as a pair, the Radeon HD 4770s' overall bandwidth of 102.4GB/s is a little on the low side. Higher-priced GPUs use a wider, more-expensive bus - 256, 448, or 512 bits - allied to high-speed memory. Where the HD 4770 really strikes back is with respect to shader power, at almost 2TFLOPS, which is over 40 per cent higher than the next best. Gigapixel fillrate is also impressive, as is the gigatexel throughput, helped by having a combined 64 texture-unit processors over the two cards.
The basic numbers tell us that the pair will do well, but combining them brings into play the possible foibles of multi-GPU usage. Triple-A gaming titles should scale almost linearly from one to two GPUs, but the same may not be true of lesser titles. Still, the HD 4770 CrossFire's vital statistics are strong.
Here are the twins in a test system:
Of course, you will need to have a motherboard that can support ATI's CrossFireX technology. A glut of AMD motherboards support it, naturally, as well as a range of Intel boards.