General :: Mac OS X Leopard Compatibility With Android Phones?

I'm sick to death of my iPhone. It's been playing up to the point that using offline apps is about the extent to which I can use it half the time - calls drop/don't come through, messages don't send & the phone doesn't receive them either, the connection to the internet (Wi-Fi & cellular) suddenly drops & doesn't return unless I restart the phone...

I'm due an upgrade on my contract, and the two options with the right plans are the Samsung Galaxy S3 & Sony Xperia T, both of which have a reputation of being decent, if not fantastic phones (one more so than the other). I definitely want to go to android rather than stay on with another iPhone (which is another possible option), but I'm worried about it's compatibility with my computer.

I own a 2006 iMac which is currently running Mac OS X 10.5.8 (Leopard), which, despite only being released a few years ago and is more recent than Windows Vista, has been declared obsolete & is no longer receiving updates for any software (Flash, iTunes, Firefox, Chrome, etc.), so I'm concerned as to what may happen if I choose to switch to Android. I'm already struggling to keep my iPhone working well with the computer because of the fact that I can't even update it's software any more.

I do intend to update the software to Snow Leopard, and possibly Lion depending on whether my mac will accept it or not (the apple store assistants didn't really know anything about the 2006 model, one of whom didn't even know that the older white design existed -.- ). However, I'd like to get my phone before I upgrade.

1) Will my computer's current software (and future software) be able to effectively sync my android? & yes, I know this will require some third-party software to assist.
2) Which of the two phones would be better? My intentions are for browsing, social networking, texting, and listening to music.