Quick question about External Hard drives

Hello there! So here's the deal-io, the HDD currently installed in my computer is a Seagate Barracuda 500 GB 7200 rpm. Unfortunately it is pretty much full, so my question is this, if I get an external hard drive could it run as fast as my current hard drive, or close to it? Or should I just go ahead and get another HDD?

This is a gaming build so I believe it matters how fast it could run. Thanks for taking the time to read this and have a wonderful day!

Depends on the external drive you get; for example, this mushkin flash drive is as fast as their internal ssd's. 

http://www.newegg.com/Product/Product.aspx?Item=N82E16820226459

If you have a particular drive you want to compare let us know