You can upgrade to Snow Leopard by purchasing the retail version from Apple or any of its resellers. Any incremental updates to Tiger (10.4.x) will be provided via Software Update, but all that Apple is supplying for Tiger is security and minor bug updates. If you want to upgrade to a newer version of OS X, you'll have to purchase the retail version of Snow Leopard.
I use OpenOffice.org without much issue on both Linux and Mac OS X, even on Windows when I need to.
If you really need Microsoft Office for the Mac, you'll have to pay for it. Check out Apple's online store or any of the Mac retailers online. You might also find it in some brick-and-mortar places like CompUSA or Best Buy.....or even the Apple Store.