I am thinking of buying a new hard drive. My current hard drive have sata 2 and speed of 5400rpm. Currently my motherboard support only up to sata 2. My question is, if i buy sata 3 7200rpm hdd, would the hdd have reduce rpm speed of 5400rpm or it is still 7200rpm?
The maximum data transfer rate on the SATA connection will be SATA II - or, more correctly these days, SATA 3 Gb/s. However, the HDD itself cannot retrieve data or write nearly that fast, so the actual performance will be limited to what the drive can do. The SATA 3 Gb/s versus SATA 6 Gb/s spec will not make any difference.
Now, the fact that you are moving from a 5400 rpm drive to a 7200 rpm drive WILL actually improve the performance. The main factors limiting a hard drive's performance are the various mechanical motions, and faster disk rotation improves data access speeds. In addition to that, it is POSSIBLE that the newer design of 7200 rpm drive has a few other features (like, maybe a larger cache size?) that add to the performance a little. So go ahead, make the change. Your system will NOT limit what the new drive can do.