I have read all of Power of Faerun except for The Border Kingdoms chapter. Hands down, it is one of the best gaming books I've read, period, for any edition and for any game, though of course you'll get more use out of it for fantasy genre games than anything else. Books like this fall into the category of products that make all of my games better, instead of just letting me have a game. Others I'd put in this category are things like Strike Force, The Marklands, and Nightmares of Mine.
If you are looking for rules for your epic game, you're out of luck. There are almost none in the book.
If you are interested in ideas though, this book is packed with them - a veritable treasure trove. I am not likely to use it with the Realms, but that didn't stop me from jotting down 12 pages of hand written notes of various adventure ideas, underlying plots and schemes, and campaign themes/story arcs. Mixed in with that are just some core changes to how I"m likely to frame churches, religion, nobility, royalty, and money in future fantasy games (again, regardless of setting or rules system). And once I finish writing down my ideas from the last few chapters it will probably be more like 16 or 18 pages. Some of the ideas were in the book, and others ones I came up with just riffing on what ifs and running my imagination wild. I doubt I'll ever be able to use them all. Note that the ideas run the gamut of all power levels and levels of political influence. Even though the book is pitched as for epic characters, just about every idea can be executed in a high and low powered version, as an underlying campaign theme, or just something to add flavor to specific adventures.
For me, that's exceptionally worth the money I spent on it. I wish there were more idea mines like it for GMs (and even for players).
Dragons of Faerun I'm not so sure of for your epic game. I have it, but haven't read much of it yet. What I'm likely to use it for is for dragon stats (which I hear have a lot of errata which is a big disappointment), and plots and schemes. I'm not interested in the timeline stuff, but rather the history and interests of the various dragons. That being said, for epic dragon related plots and schemes, I think there are enough in Power of Faerun that I wouldn't need this book to get a lot of dragon gaming in. My second choice for dragon themed ideas would be Draconomicon.
If you're trying to make the best use of the least amount of money, and you want both epic ideas, and dragon related ideas, then I recommend Power of Faerun and Draconomicon.
For a home brew I'm working on, where an ancient dragon ruled empire plays a big part of the back story, I will make a lot of use of Power of Faerun, and Draconomicon. I think I'd get more use out of Dragon Magic on a session by session basis, and would sometimes make use of stuff from Dragons of Faerun.
Edit - Regarding Power of Faerun again - I found the weakest chapter to be the one on Heralds and Marshalls. That said, with a little imagination, I figure I can use the Herald prestige class as more of an elite agent of the king by re-imagining the flavor and leaving the mechanics intact, so there's still value there. The chapter on mercantalism was a bit less content rich than I had hoped.