SATA 1 or SATA 2?
I have a Dell Studio 435mt. If I put in an extra hard drive, does the motherboard support SATA 2?
If it's got a SATA connection then you can plug any SATA drive into it. The different speeds of SATA are designed to be backward compatible - if the drive speed doesn't match the port speed they will auto-negotiate the fastest speed that both are capable of.
Occasionally a SATA II drive won't properly auto-negotiate with a SATA I port - the drive has a jumper you can use in that case to force it to the slower speed so that it works correctly.