AMD has pretty much used the same socket design for years. AM2 and AM3 aren't really that much different size wise. Where the differences lie is in the architecture of the chip itself, but I don't really know the mechanics that well and I can't really explain a lot about how that works.
If you want to avoid trouble I'd suggest contacting your motherboard manufacturer to make sure that chip is compatible before buying it. The X4 is also a 64-bit CPU, I'm not sure it will run adequately on a 32-bit OS without the proper drivers, but I was using my X6 on Win XP for several months before switching it to 7.