The primary difference between the main battery and the PRAM battery are their size difference and function. The main battery is responsible for powering the entire computer. Considering the age of your particular model of mbp replacing the main battery would be advisable if you want to avoid being tethered to a wall outlet whenever you use it. The PRAM on the other hand is a small lithium battery or clock battery and it's used to maintain the state of a small amount of memory which contains certain system settings including the system clock. The standard symptom of a failed PRAM battery is that your machine's date and time will be incorrect when you boot your Mac e.g. you'll get a message at restart that reads "Your computer's clock is set to a date before March 24th, 2001" - it will generally correct itself shortly after boot when your Mac checks an internet time server. In some Macs you will also see power management related issues if you have a failed PRAM battery. If you don't have any of the above...
Did you recently upgrade to 10.7 Lion? It appears that the upgrade to OS 10.7 has left more than a few people without the ability to scroll with their magic mouse. If you did upgrade, you may want to check this link out -> https://discussions.apple.com/thread/321... Cheers
As rj713 said, space would definitely be an issue. However, have you thought of perhaps adding a SSD instead? http://eshop.macsales.com/shop/SSD/OWC/M... Another idea would be to remove the superdrive and replace it with a sata drive. Doing a quick google search came up with MCE's optibay http://www.mcetech.com/optibay/ as an option. Theoretically, I would think you could probably also get away with an atapi to sata adapter. Just my 2 cents. Best of luck!
