Depending on how old, you probably have SATA 2 or older, but it doesn't matter since SATA 2 and 3 are interchangeable and don't affect performance. As long as you don't have something so old that it has IDE controllers you are fine, if that were the case you would need an add in card.
Is this for storage or a new boot drive. I like the WD Black Caviar 1Tb for a boot drive if you don't want to go to SSD. For a storage drive I would look at a 2Tb WD Blue or Green drive. Green drives use less energy but park the heads often so if you access files a lot it would be better to go with the Caviar Blue 1-2Tb.
I'm very sure that there is no 2TB Caviar Blue. Also, many of the Caviar Blacks are actually inferior in performance to some of the Caviar Blues, just something to consider. Don't buy a *color*, there are many different models ranging from almost brand new to several years old and being a Caviar Black doesn't necessarily mean that it's better than a new Caviar Blue. Last I'd heard, even the newest Caviar Blacks are still a generation behind the newest Caviar Blues of the same capacity and if that's still true, the Caviar Blacks are still literally paying more for less unless you go for a 2TB model.
Also, SATA 2 versus SATA 3 does make a difference in many cases so long as it is supported by both the motherboard and the storage drive. For SSDs, it can be a very large difference and for hard drives, it still matters for the chard drive's cache despite the fact that the hard drive's platters themselves often struggle to be bottle-necked by even first-gen SATA.