If you have an internal Laptop drive, that is run at 5400rpm, it will be slower than any SSD . But a good question is, if you are able to transfer the physical performance of the SSD over a USB port. USB 3.0 on the other hand seems to be pretty fast, so may be worth to try it.

If a 240GB drive is large enough is depending on your libaries. In the long run, it will be too small, I´m almost sure. But of course you could source out your most used libaries to the SSD and put the not so important stuff on the slower drive.

Have you thought about completely changing the internal drive towards an SSD ?

Booting and working on partition C from an SSD is always fun. Then you wouldn´t have to worry about the USB port.

The Crucial is a popular model. Of course there are more expensive models (Nand MLC vs more advanced types). Intel has good ones, but more expensive.

I have a Corsair Force and a OCZ, both with 128GB and Sandforce driven -no problems.

Samsung Evos are becoming popular as well now, but I have no experience with them.

If you have weird effects on Windows 7 with the Crucial, search on the web for  LTM state - a feature to save power, which is usually activated in Laptop environments. It can be responsible for nasty effects - especially known with Crucial SSDs, but also other SSDS with that controller.

It is not hard to deactivate it.