P5N32 E-SLI max mem freq in Dual Channel?

Hello All!
I have a question regardin max mem freq in dual channel mode.
Recently I purchased 1066 MHz DDR2 1Gb - 2pcs. They run nicely on single
channel mode (WinXP) but every single time I try to force them into dual
channel mode, the freq goes down to 800MHz even that in BIOS I set this
freq as unlinked 1066. Also in manual (written for old BIOS) it says,
that 800 MHz is max for dual channel mode.
Right now I have latest BIOS 1503 and it is still the same story.

So my questions are:
1. is it possible on this MB to have dual channel mode with 1066 MHz? If
yes, then how to do it ???
2. If the answer is NO which option is better regarding speed / efficiency:
- dual channel with 800MHz
- single channel mode with 1066 MHz?

Thank you for all suggestions in advance.

Best regards

Jarek