See the Nvidia excerpt below.
With a GTS450, you will be able to run BF3 with Low
setting and at 1280x1024 or 1280x720 (maintaining the aspect ratio with black borders at top and bottom). Note this is a reduction from your 22'' WS Viewsonic native 1680x1050.
With a GTX260, you will be able to run with Medium
setting and at 1680x1050 (exactly matching the native resolution of your monitor).
Frankly, an upgrade from 9800GTX+ to GTS450 is not very significant. I suggest a GTX260
at the very least.