It won't affect performance at all. It merely splits the hard drive into two sections and runs Windows from them. I would split the hard drive in two 250 gb partitions and then use the external hard drive for extra storage space if you ever need it.
You can get started by downloading EASEUS partition manager. There's a free version. Run it and select "resize" on your active hard drive, now resize it to about half and press ok. You'll now see unallocated space on the same screen as before. Click that and then create. Make it as large as possible (use all the space you shrunk from the active partition), click ok. Now click apply and reboot. It will run during boot. Now, you can boot from your Windows 8 install media and install it on the new partition you created. That's all you need to do. During boot from then on you'll be presented with a menu to pick your boot os, which you can configure on a timer and chamge the default.