Upgrading Cpu/Mobo

I am wondering about upgrading my computer to have a better motherboard and cpu, however I am not very good with computers. I am not really sure what motherboard/cpu to get, but I just need an up to date, not too expensive one that can cope with current games like Battlefield Bad Company 2 and Modern Warfare 2. I also don't know what types of cpu's and motherboards are compatible with one another, so for future reference, any input on there would be greatly appreciated. Thanks.