There are two hard drive sizes 3.5 and 2.5inch, 3.5 inch are standard drives, they generally need power in addition to the usb connection, I also do not consider them very portable. Buying a 2TB drive will get you the best deal today, ie spend $50 for 1TB drive or $70 for 2TB. An external enclosure is about $25, consider getting one with eSata.
2.5 inch are the laptop drive sizes, more expensive per gig but truly portable imo, and can run off the power supplied through the usb connection. You can scope the prices, but you can get an enclosure for 9.99.
Even the slowest drives today will easily max out your usb and likely eSata connections, 5400rpm is perfectly fine imo for storage purposes.
I have a few externals, I mainly just use them to have an extra backup of important files away from my computer, you never know when flood etc is going to happen. If speed really is an issue you maybe need to get a USB 3.0 card, or if its not perhaps consider grabbing something like NAS device, check out DNS-320, pretty useful hardware imo.