Budget??
The typical recommendation around here is to go with Infinity Primus speakers. They come in multiple sizes for each tower, center & book. This way you can piece something together even under a limited budget. Very nice sounding speaker in their price class. Great sound period!
+1 on keeping all your speakers of the same make. Id go another step further & recommend keeping them in the same series as well. The sub on the other hand doesnt need to be of the same brand. Actually most speaker companies dont make great subs. Example: Infinity. Instead, go with a reputable sub company such as, of course, HSU, SVS, Rythmik, Outlaw.
This is basic advise that, i'd bet, most here will give you.