How much space do you need? If you hold out a bit longer you could go with this bad boy: HGST Ships 6TB Ultrastar ;-}
Putting our feet firmly back on the ground here:
Your limit is the SATA I/O speed your system runs at. In your case you have a SATA II (3.0 Gb/s) port. The OS version you are running could be a limiting factor. You'll need to be running OS-X 10.x. Here's the Apple T/N on OS-X limits: Mac OS X: Mac OS Extended format (HFS Plus) volume and file limits. As you can see you're not likely to hit any of these limits (unlike other OS's) just by upgrading to the newest OS-X you could go with the 6TB Ultrastar ;-}
In any case you may want to boost your RAM to the max of 6.0 GB while you're upgrading as the newer OS's and the larger HD's need a bit more room.
You do have a lot of choices here:
- You could go with a larger HD 1 TB or 2 TB would be possible
- You could go with a SSHD hybrid drive both 1 TB and 2 TB drives are available which will give you a boost in speed booting up and running apps.
I wouldn't go with a SSD, while they are good, they don't offer the size at the price people are willing to pay. While some people have swapped out their optical drives for a second HD or SSD. I wouldn't recommend it on this model as the optical drives I/O is quite slow.
We like SSHD's they give you the best performance at the best price.
Yes, you can go upto a 4 TB SSHD drive.
Seagate offers one: Seagate Desktop SSHD. And heres the spec sheet for it: Desktop SSHD specsheet. If you look at this line SATA Transfer Rates Supported (Gb/s) you'll see 6.0/3.0/1.5 as this drive offers auto SATA port speed detection. Not all drives (HD or SSHD) offer this, some have a compatibility jumper and others are fixed and may not play nice in your system.
In any case you do want to make sure your firmware is up to date. Follow this Apple TN on how to check and update: EFI and SMC firmware updates for Intel-based Macs.
I would take the time cleaning any dust build up inside & the heat sink fins and it you can I would also clean and refresh the thermal paste on the CPU & GPU chips.