I havent run vantage on my x2 by itself, but On 4870 512mb @ 870 I got 16700.

that was a 200mhz higher CPU clock, but 200mhz is not a 1700 point difference. I think you need to raise your ram speed, 846 5-5-5-15 is terribly slow... either tighten timings to 4-4-4-12 or go to 4:5 divider to get your bandwidth up