+1 for Andre's comment!!! Who needs other stinking RPGs if you own all three of these games?
There is a snippet of my review about HQ2 in Jeff Richard's blog at LiveJournal (and flames from the fans of previous versions of HQ). The whole review is in the Glorantha discussion group
http://vingkot.livejournal.com/6854.html?view=58310#t58310
The bottom line of all is: HQ2 will not help you much if you are running a MRQ campaign as the only relevant part of it is the magic system which is not explored in detail in the basic book. It can however be regarded as a very useful collection of GM hints that will work for any system, so try and get it if you can.
Sartar, OTOH, is really, really useful and gives a depiction of Orlanthi society that is deeper than any MRQ supplement (though I haven't read BoO yet), and the few HQ2 stats in it do not "get in the way". The magic is also rune-based, so it is very easy to adapt the cult description to RQ magic, which is also rune-based.
The only problem is that Sartar will not be available in July, because Moon Design has not completed the production process yet (maps and illustrations are missing). But by all means, manage to preorder a copy as soon as Moon Design sets a release date for it! You will not regret it.