Paradigm Monitor 7's. They are efficient to run, so they won't strain your receiver too much, plus they sound great with yamaha. They are probably around 600, and a great bargain at that price. Have a listen and see what you think, also try PSB if don't like paradigm.
The Monitor 7's would be a good choice but if you're considering Paradigm, give a listen to the Reference Studio 20's which you could get for around $700. Of course you would have to add stands, but for two channel music, the 20's are very nice sounding speakers -- certainly a step up from the Monitor series.
I haven't heard the 603's, but the b&w speakers that i have listened to sound great,a very different sound then paradigm, so listen to both and see which you prefer, try and listen to both of them with a yammie as they do sound a little different with different receivers.
I also have a RXV640, and am NO audio expert. This week I bought a pair of Infinity On-Wall Speakers (OWS 1) for the rear left and right. I barely hear them at all. At circuit city they sounded great. Is it this receiver, or am I doing something wrong as far as settings...I read the manual and as I said, I am no expert. With DVDs the receiver shuts itself off, and I am not even getting that full sound. THANKS FOR ANY HELP.