Your benchmark tests look right on, given the specs of your SSDs. The X25-M G2 80GB SSD is rated at a max of 250MB/s Read, and 70MB/s max write. Your 520's results are fine too, and I can see you have a board with a SATA III chipset.
You'll get better performance using the 520 as your OS drive. At one time, the G2 SSD was one of the best you could get, but SSDs have evolved quickly over the past two years. Your 80GB G2 is hardly worthless, and faster in most respects than any HDD, but a bit weak in write performance. It would not be the best choice for use in a very write intensive environment, but has plenty of life left in it.