By the looks of it, your the type of guy who wants to do it right. You have a great start with the projector and blu ray. I would recommend the Yamaha RX-V663 over the Onkyo 606 because it will have pre-outs (for a future amp if you plan on getting serious), if you don't then your 606 is a fine choice.
Since you have a great foundation for your setup, to save money and do it right, it would be best to upgrade by adding piece by piece onto your system with the $600 budget instead of looking for the quick fix. With that said, I would spend that $600 solely on a pair of nice front speakers, it could be bookshelves or floorstanders. Floorstanders will typically have a built in subwoofer or midrange woofers which would do just fine for low frequencies until you add a sub later.
A nice sounding and slick looking pair of floor standers would be the Onix Rocket RS450's...
http://av123.com/component/option,com_virtuemart/page,shop.browse/category_id,19/Itemid,37/
They are $100 over your budget however. Another route would be looking for a pair of speakers used. You can get a lot more speaker going that route, and it is typically a safe route to go.
Anyway, search around this forum and find out as much as you can before you rush into any decision.
edit: If your looking for the quick fix, the best route to go for your money would be:
Fronts: Polk RTi8s
http://www.jr.com/polk-audio/pe/POK_RTI8_hy_CH/ -- $400 for a pair.
Sub: Bic Acoustech H-100 :
http://www.overstock.com/Electronics/Bic-Acoustech-H-100-Powered-Subwoofer/1513356/product.html .. overstock has coupons to knock the price down to around $230.
Center: Polk CSi3
http://www.jr.com/polk-audio/pe/POK_CSI3_hy_CH/ -- $100
Obviously you would pick and choose between the sub and center...