No problem - I have done that myself, even changed the CPU. The only device you cannot change (without special intervention) is your disk where the OS resides (assuming that is also the active partiton - which one never knows these days).
2. If you have a 32 bit OS installed now, and you add enough memory to equal or exceed 4 GB, you won't be able to use it all, and should consider installing an x64 OS.
Not necessarily - depends on the applications. With 4GBs, there is not much to be gained with a 64bit OS. That is different with more than 4GBs.