Try putting the original memory back in, and see if it will boot. When I first bought some memory (from Fry's) one of the modules was bad, I put the old memory back in and everything worked peachy. Went out and bought some Kingston memory from a local store I trust (bit more $$), no problems after that.
If you removed something when changeing the hard drives, that's bad. From what I saw in a video of removing the hard drive, it should just slide right out, and the new one slide right in. Make sure the connectors are the same! The MB uses SATA.