You want to pick up some outdoor rated speaker wire, which can be found at
www.outdoorspeakerdepot.com and take a look around their site which is pretty darn nifty.
I would not use anything outside, that is not rated for outdoors, and 6.5" speakers would be a minimum size.
You are fine with using a volume control to 2 pairs of speakers, but make sure the volume control can handle the power you throw at it.
Keep in mind with speakers that you have a ton of choices which includes rock speakers, partial burial, wall mounted, integrated lights, pots, and dozens of specialty versions. With the outdoors, if it is a large area, you need more speakers, often a lot more, to get good coverage without the need to blow out your neighbors. You could consider more pairs of speakers and a multi-channel amplifier perhaps, or even a surround receiver with multi-channel stereo setup.
Something like this isn't bad if you have some landscaping to put it into...
http://cgi.ebay.com/New-TIC-8-150-Watt-Outdoor-In-Ground-Speaker-/390184390637?cmd=ViewItem&pt=PDA_Accessories&hash=item5ad8cd4fed
Likewise, the nicer rock speakers sound pretty good, but some deals may be worth trying out...
http://cgi.ebay.com/4-pc-New-6-5-Granite-Outdoor-Garden-Rock-Speakers-4R6G-/330425017607?cmd=ViewItem&pt=Speakers_Subwoofers&hash=item4ceedda507
I use Sonance outdoor speakers on my deck which I have mounted to the side of my home and they sound pretty good for the few times I use them. On my front patio I use in-ceiling Sonace speakers since the patio is covered, and they sound good as well.
You have lots of choices at tons of price points, but the outdoors is very difficult to fill with audio, and bass response will be weak, so just be aware of that.