Depends on the motherboard. If your ram and cpu fits in the slots, your good to go ! simple as that. As far as I know all Am3 boards are back compatable with Am2 cpus. Just check the board specs, that will tell you what you need to know. Also, you should be able to find boards cheaper than £400 no problem.
no, you CANNOT put an am2 processor into an AM3 mobo. you can however put an AM3 CPU into an AM2+ mobo provided the BIOS supports it.
@OP yes, the ram and CPU are perfectly compatible, as long as you have a AM3 mobo to put them in, which it seems you do.