Wieland Willker has produced an incredibly helpful textual commentary on the Gospels of the Greek New Testament. Each Gospel has it's own pdf where Willker walks the reader through all of the disparate evidence for the textual variant and then he suggests a preferred reading. The best part about a commentary is that you don't have to trust the author's reading: all of the evidence is there for you to come to your own conclusion.
It looks like he's already updated it for 2012, and the full commentary comes out to approximately 2,600 pages and 2,000 variants.
You can look at the textual apparatus on your NA27 to give you a quick idea of what is going on, but for the important variants, I would check out Willker's commentary.
Going further down the textual criticism rabbit hole, you can review all of the different papyri evidence for the GNT and then, if you so choose, look at the actual papyri images.