According to ASUS, your board "does support" the x6 CPU's with BIOS 0804 http://www.asus.com/product.aspx?P_ID=KWQH5MxS6HH3nWjy
However if you decide to go with the 1055 getting to @4.0 probably won't happen. Having the multiplier locked-down means you will be clocking via the FSB. As you increase the frequency (ie... from a base clock of @200 to let's say to...@220) you are increasing the speed of other components as well, which can/will limit CPU speed.
If you can, go for the 1090. Having an unlocked multiplier should make it way easier to get to @4.0 And it looks like prices are dropping, if it where me I'd keep watching to see what happens in the next couple of weeks as X-mas sales are on the horizon.