Your base mnagic resist is 0% (when you start anyway) and wielding Carsomyr should set the base magic resist to 50%. However anything else which ADDS magic resist will raise it above it. For example you can get 55% magic resist by wielding Carsomyr and putting on the necklace you get from Unseeing Eye (+5% Magic Resist). Add to it another necklace - forgot the name but you get it lateron in the game - which adds +10% magic resistance and it's at 60%. Add to this the +10% magic resist you can get when doing the Tears Of Bhaal quest much later in the game and it's 70% magic resist. pretty handy. And there's more items with magic resist on them I think, need to recall the names, and you can get it up even more.
I currently have 55% magic resist from Carsomyr and the necklace and it's pretty neat.
EDIT: Btw, you may also want to get a fix from
Baldurash for Carsomyr. By description the Carsomyr is 1d12+5 sword, but due to this bug it's actually 1d10+5 in the game. The fix from Baldurash will set the Carsomyr to be 1d12+5, as it should.
[ 12-19-2001: Message edited by: Whailor ]