If you already have a receiver, I would not bother with a new one unless you really need it. Surround receivers are like computers, in that they lose their value rapidly, and next year you will probably be able to get something better for the same amount of money. Also, speakers make much more difference to the sound quality than the receiver generally does, so you need to put most of your money in your speakers.
So, if you already have a receiver that is adequate, I recommend putting ALL of your money in new speakers. (Remember, subwoofers are speakers, too.)
Listen to the speakers before you buy, and choose the ones you like best. No speaker is perfect, so it is a question of which defects one finds least objectionable. For probably the best bang for the buck, getting bookshelf speakers all around and a really good subwoofer is probably your best bet. Because the deep bass will be handled by the subwoofer anyway, there is no point in paying for really deep bass capability in the main speakers. The bookshelf speakers do, however, need to go low enough to properly go with a subwoofer, and generally it is a good idea to get satellite speakers that are solid down to at least 80 Hz, so a -3 dB point of about 60 Hz or so would be good. Of course, for it to sound right, you must set it up properly, too, so read the manuals carefully of the equipment you will be using.