Your PC is going to need ALOT more to run BF3 well. The Inspiron 546 is an entry level home multimedia PC at best. You'd need to upgrade alot. The Video card is an issue, as the integrated card you have is pretty weak. You'd need to upgrade the power supply before you did the vid card, which is a pain on a Dell due to their proprietary cases. You can mount a PSU outside of the box though.
Tac was saying that ATI sucks... He's wrong... Also, just popping a new graphics card in and hoping for the best is a recipe for disaster. If you're lucky, your PC will just run like crap. Worst case, you underpower the MB or your new card and fry one or both.
Your best bet is to start over. You can build a reasonable gaming rig for $700 or so if you have all of your accessories (monitor, mouse, keyboard, etc.). You'd be MUCH more satisfied with that.
If you HAVE to use that rig, I'd start with a good PSU, then GPU. That way you won't ruin what you already have. I have my doubts as to whether your MB can get the most out of a high performance GPU though. Most likely, you'll still have bottlenecks on the MB that you won't be able to overcome.
Good luck.