if you're looking for both quiet and speed, then it's hard to go wrong with an SSD. You can still have an HDD as a secondary drive for your data. You'll want at least 128GB, but 256GB are becoming more affordable. This'll cover your OS and your apps with space to spare. I like the Marvell controller based SSD's (Crucial M4 is reliable, and Samsung has their own in-house MCX and newer MDX controllers (the tried and true 830 series and the recently released 840 and 840 Pro).
A 128GB can be had for around $110 ($70 if you get it on sale) an 256 at $2000 (The Crucial M4 is on sale right now at Newegg for $170). The Samsung's are faster than the Crucial and the 830 has a proven track record as well.