disable your virus programs to have a chance at fighting the act 2 boss with no lag
run the game on a flashdrive to help with stuttering
roll a WW barb
dont spend any money on the RMAH the stuff you buy will likely be nerfed
annnnndddd finally get a return and pick up Xcom.
Act 2 boss lags because of a sound error. It's a known bug and will be patched in 1.0.6.
The game runs fine on a mid range system. Make sure you have the latest video drivers.
Have fun coming up with a build that suits your play style. The initial difficulties are very easy which allows you to experiment with all the skills as you unlock them. Once you get to level 60, you should be quite familiar with the character class you're playing.
Don't spend money on the RMAH, you will find it much more rewarding to find your own loot.
and yeah, check out Xcom: Enemy Unknown at some point. It's good (and so is Diablo 3).