Try cpu 840 + mem 1350 and -I 20 (you won't be able to do anything else on your box with -I 20 though)
The key (I think) is gpu vs mem clock ratio and making sure the driver doesnt adjust the clock down because it's getting too hot. You should be able to set a higher TC though. Latest drivers?
I'm getting ~810KH/s ... I don't have the actual config on hand, since I'm at work. I got it to 880KH/s (with higher clocks), but then there were glitches... No HW errors in cgminer though, but it made me nervous
I guess the memory needed more voltage, but I couldn't get it to work stably.