After reading the replies so far, I have to ask - how big is the garden and how considerate of your neighbours are you? - because the answer to those two questions could save you much cash. This group is tremendous and I have a lot of respect for the very knowledgeable people who frequent it and offer very good advice - but I can't help thinking that, on ocassions (this being one of them) things are taken a bit too seriously.
First of all, you're going to be playing music into the open air, which I would imagine is never going to be "acoustically brilliant", so I would question the need to bother (specifically) about woofers, tweeters, baffle boxes and so forth.
Secondly, unless you have no neighbours nearby (or just don't care about your neighbours), you'll never be able to play your music very loud so you don't need to worry about power levels or cable sizes.
We live in an "average" semi-detached chalet-style (dormer) bungalow on an "average" housing estate. We have an Onkyo TX-SV343 AV Receiver/Amplifier under the telly that has outputs for a second set of speakers, so I bought this
Pete