We haven't found a GPU miner yet that can handle that N.  Most appear to allocate the hash scratchpad from the stack instead of the heap and die with stack overflows.  Good news: that is fairly easily fixed by the code writers if they wish to do so.  Bad news:  we are waiting for it to happen for some someone to find a branch that works.

When they do, I'll post, since I have half a dozen GPUs I'd like to point at it.

Good time to point out that midrange GPUs are hashing at, ballpark, the same speed as CPUs.  e.g.  My GTX 560 Ti is doing about 90hashes/sec which straddles various CPUs I have running (40-120 hashes/sec).

Cassey