Suggest New Internal Hard Drive
I just upgraded my Macbook Pro hard drive going from 300 gb to 1 tb. Always go for the biggest you can afford. Get a portable kit with the new drive. Then when you upgrade you can put the new drive in the portable kit, clone your interal old drive and then swap the drives, old in the portable enclosure, new cloned drive in your laptop. Then you have a backup of the old just drive just in case and ultimately can erase the old drive and use it as a portable.
I'd also suggest getting an external drive for running Time Machine.