Yeah, I say you'd be good at those settings. If you stick with that resolution. However, I can see it running at low-med at a higher res (probably up to 1680x1050).http://www.tomshardware.com/reviews/battlefield-3-graphics-performance,3063-3.html
This benchmark here shows the 450 will run BF3 at high on 1680x1050 at 30FPS. You'll be okay (this is a SP mission, but at low-med and a low res, you'll be more than okay, and can even get a bigger monitor!) I'm going to say yes to 50FPS+ on your res at low/med.
Also, you do not need to add anything to get the game to run as you suggested. However, run Windows updates and get to at least SP2.
How much RAM do you have by the way? You might need to increase that, though, if it's below 2gb.