Yes, all versions of SATA (except the first for obvious reasons) are 100% backwards compatible.
So the 1TB drives will work with my MB just slower I assume.
Thanks for the answer
That depends on the drive (specifically the spindle speed 5400, 7200), You wont notice and real world difference though.
Most Hard Drives aren't fast enough to saturate a 1Gb/s connection, let alone 1.5/3/6.
It is supposed to work perfectly - the SATA 3.0 Gb/s or 6.0 Gb/s units are supposed to figure out what their controller is doing and adjust themselves to that automatically. In a few cases this fails. If that happens, check your HDD manufacturer's website for the way to force your newer SATA HDD to slow down to original SATA 1.5 Gb/s communication rate.