It has NVIDIA GeForce 6100 / nForce 430 chipset which supports socket 939 CPUs so any Socket 939 would be fine(make sure it fits in the case though) but they are hard to find now since they are discounted
Your system probably uses a standard micro atx board. But your vista operating system may not reload with a non acer board or microsoft may not activate it. So if you change boards, have a backup operating system available if needed. I see one spec sheet that shows you use an am2 cpu, but use cpuid if possible to positively id your cpu.