CPU makes very little difference in BF3. It is almost entirely limited by GPU, and the 550 will get owned on anything more than medium at HD resolutions. Also you have it the wrong way around, the lower the resolutions the higher CPU workload.
Windows 32 bit can address 32^2 memory spaces which is 4GB. Thats total. So if you have 4GB of RAM installed your system will only be able to use the 4GB minus whats on the graphics card, HDD, sound cards etc. So usually around 1.5GB in your case - not really enough. You would need a 64bit OS to use the whole lot.