If you're buying a HDD, you might as well make sure it is SATA II with 3.0Mb/s and NCQ.
The performance differences aren't huge, but it is worth your while to take note of. Prettymuch all of the performance difference however comes from NCQ (Native Command Queing); which is only found on SATA II drives so make sure it says it has NCQ.
Here is an early comparison between two seagate 7200.7 drives, one with NCQ and one without:
The reason the 3.0Mb/s spec doesn't really mean much in terms of performance is because current hard drives can't really even barely ever hit the SATA standard of 150Mb/s. The drives just physically aren't fast enough yet to utilize the large ammount of bandwidth offered with SATA and SATA II.
But long story short, just get a drive with NCQ and 3.0Mb/s and you'll be all set
I believe they should all be backward compatable but if your motherboard chipset doesn't support the SATA II standard, I don't think NCQ will work. For instance, the nForce4 Ultra chipset supports SATA II and NCQ while the nForce3 250 chipset only supports SATA so I don't think NCQ would work on the nForce3 chipset, but should work fine on nForce4 Ultra.
Sorry, I'm not up on my intel chipset specs