Real answer: depends on what you're using it for. There isn't just "fast", there's fast for any number of applications. And they're not linked, that's right out. Tom's does good research on that stuff. http://www.tomshardware.com/storage/index.html
Or you could check the storage review performance database. Seems like a good place to start at least, and probably as much info as you're ever likely to need. They have a really good review on the current generation of 250's all head to head too.