obligatory introduction:
Hey everyone. Been browsing the forums for the last few weeks. Thanks for all the great information

Long story short; I just moved into my own place for the first time and now I need to build my home theater. I have a few speakers from friends and family that I scrounged up as temporary fixes while I work on getting what I really want.
the fun stuff:
Here's what I have.
JBL Northridge series N26ii x2
Polk Reference Monitor Series RM3000 x2
Polk CS100 center
Are any of those worth keeping or integrating into my system, or should I just sell them off to put them towards other components?
I'm trying to spend less than $800 for everything and I already spent $400 on my onkyo tx-sr650 just the other day. At least this is my 'right now' budget. That isn't to say I will continue upgrading over the next 6 months if i feel the need arise

Main things I want to upgrade now and the front and center.
I was looking into possibly the Polk R50 for my fronts, but I can't find the towers in black for that silly cheap price at Fry's lately. I'm also tempted to order the Cherry and then make them black myself. But I've been hunting craigslist etc for deals on some black R50s in case I get lucky.
For the center I am definitely baffled. I have my TV mounted over my fireplace, and there really isn't a great place to mount a center. I wanted to avoid the floor if possible and I don't think I want to go even higher and put it above the TV. This leaves somewhere between the mantle and top of the fireplace. I'd post a link to pics but apparently I need some more posts before I can do that. But I've been browsing around trying to find a somewhat sleek designed speaker (less than 6" deep) I can possibly mount to the bottom of the mantle.
For the rears, I'll probably use those Polk RM3000s I have for now and upgrade those at a later time.
For the sub i was looking into a Hsu STF-1 I think. But my friend might be selling me his old Polk PSW110 for cheap though so I'm holding off on the sub for right now.
This system will primarily be used for movies and games, but I also listen a pretty wide variety of music. I have XM radio that will be the primary music source and my old school record player secondary

The room is an open design into the living/dining/kitchen. Hard to post exact dimensions without a pic of the floorplan.
Thanks, and I look forward to any comments

Omni