I was having the same predicament just a while back today, and it came to my attention that really, that 2TB drive your thinking of buying isn't much faster than your 640GB external. See there aren't many ways that manufacturers can increase the speed of a Hard Disk drive other than increasing the platter spin rates (RPMs) which doesn't yield a noticeable return on Read/Write speeds. For example my computer now is using 2 old Seagate Barracudas from 2007, so I decided I'd get a faster one, and it turns out, the same drives are still made today, at the EXACT same read and write speed they were back then, they only increased the Storage space is all.
That being said, because HDD's have hit that technological barrier where they really cant get much better, SSDs are the jump in performance that you are hoping you will get from a new HDD. But i'm telling you now you wont get that performance boost from an HDD.
I refer you to this image that was shown to me, a chart from anadtech. Btw if your ever looking for reliable info on Benchmarks and testing of new tech, anandtech is the way to go.
http://images.anandtech.com/graphs/graph5729/45464.png
So as you can see by opening up that chart, even the stronger WD raptor drives, which cost around $400-$500 by the way, Fail in comparison, with only one third the power of the entry level SSD, which costs about a half or less than what the Velociraptor costs.
Now with allllllll that behind us, I recommend this drive as it was recommended to me, and really its got nothing but good reviews.
http://www.newegg.com/Product/Product.aspx?Item=N82E16820148449
That is the same drive I myself will be purchasing and as luck would have it, its on sale for $50 off right now. I will be using my 2 x 360 Seagate drives as backups, which is what I suggest you do with your external drive. What would be wise is to install your OS and all other priority files you have to the SSD so that they may benefit from the greatly improved speeds, and keep everything else thats not called upon frequently or that wont benefit from increased speeds so much (Movies, Music, Images, Less frequently used applications, etc) on your backup drive.
Hope this was informative for you, best of luck.
PS. If you wanted a smaller SDD, the same brand and model comes in 64GB and 128GB versions. However I dont recommend it, as after Windows and the Partitioning of the SDD take their 30-35GB off, your not gonna be left with much. Besides the bigger you get it the more programs you can have benefitting from its increased performance. And If price is an option, I suggest giving up now, as Any 1tb-2tb drive that fits the description of what you are looking for, will cost the same price or more as this SDD.