<font color='#000000'>seabiscuit, I use a similar Yamaha model -- the RV-1105. It was contemporaneous with the RXV-595A and -795A, rated at 5x85 WPC, which I believe was the -795A rating. It's great for my HT and music needs, and from what I've heard and read, the gains from 5.1 to 7.1 are not that great, especially since at present few DVDs make much use of the additional presence and/or surround channels. I've even read posts where people bought the 7.1 receivers and added two new speakers only to say the changes were negligible and they wish they'd spent money elsewhere.
However, does your RXV-595A have a variable crossover setting for bass management? My RV-1105 does not -- it has a fixed crossover, set at 90 Hz. (You can check the specifications online or at the back of the manual for a description.) One good reason to upgrade would be to purchase a receiver with variable bass management, such as the HK 525, Yamaha RXV-1400 or -2400, or some the Denon AV receivers. There are several great articles and FAQs on this topic on the Audioholics site -- just do a search for "bass management" and/or "LFE setting."
That said, if you're happy with your system, I would not touch the receiver, but would save money instead for di-pole surrounds (or the Axiom QS-8 quad-poles), a new universal DVD/CD/DVD-A/SACD player (and high-res audio titles to go with it), or an HD-TV. Those will all bring your more satisfaction than the cost of a 7.1 receiver AND two more speakers. In fact, even if you upgrade the receiver for better bass management, I'd still skip the new channels and put money elsewhere than in 6th and 7th speakers.
My .02.</font>