CPU intel Dual core E5500 2.8GHz
VGA: NVIDIA GTS450
Is it able to play??? Low at 800x600???
You have a good GPU, i don't know why you say it's "so bad", you should have no problem running the game even on 1920x1080p with no problem, i have a HD 5570 for testing, but a little bit better CPU (a quad core 3.1GHz) but that doesn't give much fps, i run it on 18-20min - 24-26fps average and 45-60 max fps (don't get me wrong i only get this much when i'm playing the flying mission, mostly high fps i get 35fps).
Here's what you have to do:
Try updating the drivers, the Nvidia drivers, i herd they have a special Drivers for Battlefield 3 for 30% fps increase, next try scanning pc for Viruses the could cause fps decrease... download the tool CCleaner and scan for Registry errors... if that doesn't work try format PC install the windows again, maybe it'll work then, if that doesn't work i don't know what else... I think you'll finds this helpful...