Hard drive speed is limited by the types of files you are transferring, speed of drive, if you are performing other tasks (or the computer is doing tasks in the background like indexing), and the speed of the interface.
One giant RAW video file will transfer a lot faster than 100,000 text files taking up the same amount of space.
That's a pretty slow HDD to begin with, maybe not 20MB/s slow, but my 7200 RPM 320GB drive gets around 70-75MB/s, which should be considerably faster than a 160GB 5400, due to both rotational speed and data density. You may just have a slowly dying HDD, so it may be a good time to upgrade it.
1. Is a reformat able to get it back to 50~ MB/s ?
2. Is the SeaGate Momentus XT 500GB Hybird 7200RPM good?
3. Is the Intel 302 SSD (80GB) good? Will it have problems with OS X? Or just go with X-25M?
4. Which should I get?