there is ive read a 6% performance increase with the 90nm but if your building right now i would go 130 nm, as it will give you far less headaches, i know this from my own recent experiences.
and with the abit boards flashing bios is impossible unless you have a 130 nm processor that will post so you can flash, so therefore the 90nm turns into a paperweight unless you have a 130 nm processor laying around or someone who does and can flash your bios to the latest version.
the only board ive heard of that can flash w/o a 130 nm processorr is the asus - av8 (ASUS not ABIT)
all in all id say go 130 nm for the time being, or wait it out untill you can get a board that comes out of the box ready for 90 nm processors.