Some questions to get the ball rolling: Do you have speakers and/or a sub now? Do you have a receiver now? If so, what do you have?
[Also, are you talking about the PN50A650? Just a heads up that it's a bit over $1400 at Amazon right now.]
If it were me, and I was starting from scratch with no gear, and I only had $500 to spend on a receiver and speakers, I definitely wouldn't be trying to get audio over HDMI at this point. I'd go with something like the Onkyo TX-SR506 (which I've seen for $150, but it normally goes for $200). I don't have a PS3, but I think that it has a digital audio output on it - you could connect that to the 506. You wouldn't get the new lossless codecs, but they would be pretty worthless on a $100 set of 5.1 speakers.
The 506 can do Dolby Digital, DTS, and so forth.
Audio over HDMI, and processing of the "new" codecs will get cheaper and cheaper. Heck, the Onkyo 507 might have it if you can wait until May, and it's street price will likely start at $250 and drop in a couple of months.