Hello..FSB is front side bus. This is the bus speed of the memory. There is obviously going to be a speed increase with the A7A266 board over the A7A133. That is pretty much it. DDR is going to be the way of the future so I guess your fighting obsolescense with the A7A266.
I think you mean the A7V133 ( V = VIA ) based on the KT133a chipset. It's basiclly a KT133 with added 133mhz Athlon FSB support. It still uses PC133 SDram.
The A7A266 ( A = Ali ) is based on the Ali Magik which supports SDRam & DDRam ( DDR works better of course ). Here's a review of it -> ASUS A7A266 - ALi Review. There's a new revision of this chipset which places it near the KT266a in terms of performance.
In case you were refering to the A7V266 ( Via KT266 ) or A7V266-E ( Via KT266a the one I have :grinthumb ). They only support DDRam.
If you're looking forward to an upgrade, I highly suggest you take a KT266a based mainboard. The KT333 ones are coming out thus making the KT266a ones a bit cheaper. The KT333 probably won't be taking off until the Athlon upgrades to a 166 FSB.
I thought there was something wrong with the A7A thing...Didou is correct. If you want to get the latest and greatest go with a KT266a based board. They are the fastest yet. MSI seems to have a very good model with the K7T266a.