I encountered a similar situation before. When I added 1GB to the 512MB that I already had, it underclocked itself to DDR333.
I benchmarked with 512MB(normal speed), 1GB(normal speed), and 1.5GB(reduced speed). I found that both the 1GB and 1.5GB configs gave me better performance than the 512MB config. But, 1.5GB had very close to the same performance as 1GB.
Your system might be different and respond to slower RAM better than mine did. I would try both and benchmark.
By the way, your sig is too big. I would change it before the SigSanta pays you a visit