Yes, you will notice a substantial upgrade in graphics, but BF3 on max will take a CPU upgrade. Look at some of the GPU and CPU review, BF3 is one of the most common tests.
Yes, the higher the resolution the better graphics card is needed, but that's a small piece in the larger story
So yeah, it 's a good idea to upgrade, you see a big improvement right away, and another improvement when you upgrade the rest of your system
edit: the PCI-E 1.1 is a minor issue compared to the others, and will be resolved when you upgrade you cpu since you will need a better motherboard which will come with 2.0 or 3.0 (virtually equal at this point)