SATA2 spec is 100% backward compatible with SATA controllers. Therefore, you should be able to use a SATA2 HD with your motherboard but wouldn't be able to benefit from the speed boost that SATA2 offers (unless you can somehow upgrade your motherboard to support SATA2 as well).
Worst case, if something goes wrong, you will need to disable the SATA2 features of the HD which will typically be possible using control software (device drivers) available from most HD manufacturers. However, most likely this won't be required at all.