I'm afraid you won't get anything like double the speed.
The drive mechanics for IDE, SATA and SATA2 drives tend to be identical, the only thing that changes is the controller and interface.
That can make a difference in speed, but not a massive one (except for the fraction of a second it takes to empty a drives cache), the main difference with SATA2 is that it supports a function that can reduce the amount of moving around the drives read/write heads have to do thus speeding it up a bit (basically it lets the drive finish dealing with one file before moving onto the next, rahter than trying to read/write 2 or more files at the same time).
Remember when looking at drives, you want sustained transfer rates (what it can consistantly transfer from the drives magnetic disks), rather than "burst" which is how fast it can transfer from it's cache ram to the host.
Very few drives can really max out even UDMA 66 consistantly, and that is a much older standard than SATA2.
