First off, unless you are set on buying those items locally, I would shop around online as you can save several hundered dollars between the purchase of both of those items, which could lead to you saving money, or putting the savings towards a bigger tv, or better receiver. Check out onecall.com and amazon.com for the tv, and the receiver as well, but also check out jr.com vanns.com and some of the other online authorized Yamaha dealers. But to get 7 ch. audio from your sources, use HDMI for the PS3 and set the receiver to PLIIx that will give you 7 ch. audio regardless of the source material. I got my RX-V1800 on Ebay and saved about $300 over retail, but the only downside is that voids the warranty, but to me the savings were worth it.
As far as your second queestion goes, most subwoofers are powered, so you won't see a power rating for subs, it will just have a subwoofer preout that you connect to the subwoofer. The sub will have a built in amplifier so you just connect it to the receiver and adjust your level and crossover.