1066MHz DDR2 memory can run on Abit IP35-E, for sure.
Currently I have OCZ sticks (http://www.ocztechnology.com/ocz-ddr2-pc-sli-ready-edition-dual-channel-eol.html)
running @ ~1080MHz on my Abit IP35-E. That setup works fine and stable.
I have tried also Kingston HyperX sticks (http://www.tomshardware.com/reviews/4gb-memory-overclocking,2024-5.html)
and I had 2 x 1GB sticks running stable at ~1070MHz with latency timings of 4-5-5-12... with Intel Q9650 @ 4GHz.
That motherboard (and IP35 Pro) must be one of the best boards for LGA775 overclocking. There aren't many deep clocking options in BIOS, but still enough to reliably pump things up. And then that board just seems to keep going like a train.