SATA1, 2 and upcoming 3 are all compatible. The speed will be limited by either the hard drive (or SSD) or by the interface. For example, a future hard drive running at 400MB/second in SATA1 will be limited to 150MB/second.
If you have a SATA2 (300MB/sec) connector on your motherboard, in 2 years when SSD's are more reliable and cheaper I'd get one for my operating system but spend your money on size and quality and don't exceed 300MB/second which would be a waste of money. (a 300MB/sec SSD is incredibly fast, especially when combined with 4GB of RAM to buffer your programs)
Wikipedia should be your first stop for questions of this sort anyway.