GOS, the only thing you really need is more RAM.
Selling any PC with 4GB of RAM is just wrong.
Not knowing the brand of your PC and the MB, you can either buy another 4GB stick for 8GB or an 8GB stick for 12GB which would be the way I would go.
Right now, you don't have the RAM overhead to load many of apps into memory.
Win 8 takes about 3GB at idle.
Another way to boost performance would be to add another 500GB SSD drive and run both in RAID 0 config if your PC has a second SSD bay and interface.
This will essentially double the SDD throughput. Ideally you want to use the same brand of SSD and if possible the same model although that may not be possible.
If you're not running Win 8.1 Pro, I highly recommend that you do and if you don't already have it, there's a freeware called Classic Shell that allows you to modify
the Windows desktop to anything from XP to 8.1 to custom mix. Highly recommended as well.
I already have an external drive with half a TB connected. Is that what you mean? Also, I already run Classic Shell.