I'm thinking upgrading my Server USB drive to a SSD where it will be installed Ubuntu 12.04 and i was wondering if there will be any improvement over the USB ? Thanks
It depends on what sort of USB connection you are talking about. If your enclosure only supports USB 2.0, then the maximum speed you can talk to the drive will be 60 megabytes a second. This is the maximum speed: if anything else is using that USB bus, then performance may be lower.
To put this in perspective, OCZ sells solid state drives with maximum speeds ranging from 250 megabytes per second (a slow, high capacity drive) to 550 megabytes per second (a high performance, lower capacity drive).
So a USB 2.0 enclosure is going to greatly constrain the performance of almost any SSD. If you can't mount the drive internally, then an eSATA or USB 3.0 enclosure would be a better choice (assuming your system supports one of those connection methods.