If you care to read this review of the board:
http://www.anandtech.com/mb/showdoc.aspx?i=3129&p=1
you'll find that some straps work better with memory overclocking rather well while other give not so good results.
The 266/667 and 333/800 strap/dividers seem to give the best fsb/mem performance and overclockability ratio so perhaps you should try them.
I tried the same kit but with 2x512Mb and i got 600MHz CL4 on 2.75V on 4:5 strap 266/667:
http://www.xtremesystems.org/forums/...postcount=3910
2x1Gb kit because of higher density of mem chips will perhaps need more voltage so expect a bit more needed if you get near 600MHz CL4 on 2.75V as i did.
Hope it helps.