Here is a link to some shots of the wall I built in front of my house back in 2004 and further towards the end of the album there are shots of the work I've been doing since last fall on the back section in the back yard. The weather turned bad so I'm now in a holding pattern but at least I can let the fill settle and compact so that this spring when I finish filling the area it will not sink down as much.
http://home-and-garden.webshots.com/album/551434996yQeXVf
The company I got mine from locally was from Lemay Block (lemayblock.com) . I don't know the brand they make but they have a lot of variations on colors, sizes, and styles. I got the 6" x 18" and if I remember right they weighed about 68 lbs each.
From what I understand is you want a concrete mix with the higher PSI rating. The lower the number the more water that can get inside and then freeze and eventually break down the block to a pile of rubble. I've seen some walls here locally with stones supplied by different suppliers and they are slowly disingrating.
I did my own work but got guidance from two freinds in the retaining wall business. The biggest thing is a solid/level base. After this all the stones stack up. Put the correct base and thickness under your first course of base block and spend the time with a level making sure it's right. Also pay attention to your seams so that every other row the seams line up to each other (makes it look good).
This fall on the side of the house it took me 8 hours or so to get the base stone just right but the next day it only took a few hours to make the wall 4 courses high.
The stones I have are made two ways. One is a base block with the top and bottom smooth and a builder stone. These have a 1" tab on them to set them back 1" from the stone below so the wall is staggered backwards into the fill behind them.
Like anything else try and get all of the stone at one time so the batch color stays the same. If you buy some at one time and then more another time the colors may be off some.
Jim