At least ArtForz was mistaken about Cell earlier

Just let's do some simple math. Playstation3 has 6 SPE cores, each clocked at 3.2GHz and 25GB/s of total memory bandwidth. Calculating one hash needs approximately 434176 ADD/ROL/XOR operations on 128-bit vectors in the performance critical part of salsa20/8 which are executed in the even pipe (shuffles and the other instructions are executed in the odd pipe). Also calculating one hash needs 256KB of memory bandwidth (128KB is written sequentially, 128KB is read in scattered 128-byte chunks). So taking into account that SPE core can execute one instruction from the even pipe each cycle, the theoretical performance limit based on computational power is (6 * 3200000000) / 434176 ~= 44.2 khash/s. The theoretical performance limit based on memory bandwidth is 25GB / 256KB ~= 95.4 khash/s. There is a lot of headroom for the memory bandwidth and arithmetic calculations are the bottleneck. Though Cell has precise control over memory operations by scheduling DMA transfers and can overlap DMA transfers with calculations. This allows to utilize memory bandwidth very efficiently for scrypt algorithm.

This page seems to say that HD 6990 has 320GB/s of memory bandwidth. And here ArtForz tells us that it is possible to achieve < 20% peak BW with GPU. Doing some math again, we get 320GB * 0.2 / 256KB ~= 244 khash/s. Looks rather believable to me.

edit: corrected HD 6990 memory bandwidth (it is 320GB/s and not 350GB/s)