I have an internal hard drive from an old laptop (WD blue SATA 1.5Gb/s) and am looking to buy a case for it. I looked at a few and was wondering if my money would be wasted on USB 3.0 as the hdd is too slow for it and i would get the same speeds as USB 2.0
USB3 will probably get you about twice the data speed of a USB2 connection system. USB2 has an upper limit of about 480 Mb/s data communication speed, or roughly 50 MB/s. USB3 is nearly ten times that. With a USB3 connection, the actual limit will become the speed of the HDD itself. Now, SATA I has a max data communication rate of 1.5 Gb/s or roughly 150 MB/s, but a typical HDD of that era could actually deliver (because of limits on mechanical movement of heads and disks) 70 to 80 MB/s average sustained data transfer rate. That's about twice as fast as the limit of a USB2 bus speed, so going to USB3 will certainly be faster.
Looking at it a bit differently: the USB2 interface is certainly slower than any SATA drive itself. USB3, on the other hand, is much faster than even the most recent mechanical (i.e, spinning disk) HDD's, so it lets the HDD work as fast as any internally-mounted drive.