You don't really need a SSD. Yes, programs will load a little faster and load screens will be a bit quicker too. Vista is better then XP, Windows 7 is better then Vista. Since you already have Vista SP2, I'm guessing, there isn't a reason to go back to XP.
The only reason really to have XP at this point is for older games, for example a dual boot system.
The question of your CPU and GPU, they are pretty much average now but BF3 should be fine, just not at the highest settings.
Happy gaming, the Prisoner.
Edit: I looked at the specs for BF3, it doesn't look like it supports XP 32 or XP 64.