Fortunately, the Sarranids have been beaten bloody by every other faction so they're hardly a threat and I've been able to drive of the Rhodoks pretty well so far.
They're both at war with me, along with the Nords for some reason, and there's a Sarranid castle bang in the middle of my territory, with too many troops for me to take (200+, I grabbed the city and castle after they'd already been taken a few time by other armies and had less than 50 each). I've got my own faction now, the Calradian Dominion, with a city, a castle and a couple of villages stuck between the Sarranids and the Rhodoks.
I picked up Warband a few days ago after having had the original for a few months, and things are starting to get heavy.