I swapped from a Samsung F1 (which is a good Sata2 drive) to the 60gb OCZ Vertex.
My pc (for OCUK) is slow, yet it has made a good difference, not as a much as I expected, but then again i don't have a 4ghz i5 or i7.
Ignoring your dvd drive, the hard drive is the slowest part of your pc by a magnitude of 1000s.