Why on earth would an external hard drive speed up anything? If you need a ton of storage, a backup drive (say for Time Machine, extra music/videos/etc), or just want some insurance against hard drive failure, get an external drive.
But buying one and hooking it up to the Dell or a new MBP isn't going to do anything to make them faster, and it certainly won't fix a year old Dell that's breaking down.
I've yet to buy a PC notebook that has lasted for over a year. It seems like the day that year warranty expires is when they break down. Hard drive failure, hardware issues, etc. That's why I switched to a Mac, which has been going strong since I got it.
Take the money you'd spend on a hard drive and put it towards a MBP. It's a much better investment, as I guarantee the Dell will only get worse.