I tend to like OWC (macsales.com). They specialize in Mac. Additionally, for something like $99 they will actually perform the installation of any internal components that you buy from them for the Mac Mini (I dont know if they do this for other models). I think this flat price includes more than one item at once (eg. RAM and hard disc).
In my opinion one cool aspect to the Mini is that you can get the data doubler kit to add a second internal hard disc. So if you are looking for speed you could install one SSD for OS and applications and also a large hard drive for data storage. Both internal is nice so you dont have to worry about external busses like USB or secondard power supplies. Also, on the internal SATA bus then the drive will be more efficient. Something to think about if you really want speed.