Advise me on the next step

[lakingshockey]

So I've been on an upgrade kick the last month or so. I recently purchased a new 50" Panasonic plasma and a Paradigm CC-190 center channel.

My audio setup right now includes:
Marantz SR-4002 receiver
Paradigm Atom Monitors
Paradigm CC-190 center channel

I'm thinking of making one more addition. I'm torn between spending the money for a new set of front speakers (likely Paradigm Monitor 7s) or on a subwoofer (I've been looking Lava and HSU subs). If I buy a sub, I'd like to keep it under $400 and if I buy new fronts, I'd like to keep it around $600

I'm looking for any input that you guys might have. I can only afford one and not both unfortunately. Which will make a bigger difference? (My living area is approximately 12' x 12' by the way)