From my experimenting....
You should aim for 1400 or 1600, probably not higher with that RAM. You don't mention the latency, but let's assume it's 7-7-7-21 at default. You can probably leave it the same for 1400mhz, but for the jump to 1600mhz you'd need to increase the timings to 8-8-8-24.
Anyway, basically at any given timings and voltage, there's a max stable speed you can achieve. I would say an easy thing for you to do is set 200 base clock, 4ghz on the CPU, and 1600mhz on the RAM at increased timings.
On basically every system prior to SB, RAM was definitely a hold-back to OCing as it was linked with FSB (or in this case base clock) which is why overclockers like to buy 1800 or 2000mhz RAM.