How can I upgrade from Tiger to Leopard

Hello all, hope you can help me.

I´m running Tiger Last Vers on a white MacBook with 1.83 GHz, 55GB Hd and 2 GB RAM.

The thing is that the iPhone 3gs is running quite well (not last version updated, but at least, it runs) and I recently got an iPhone 4, that is too new for my Tiger.

Thus, needs an upgrade, but the Apple Store only provides the Snow Leopard.

So:

.- Can I upgrade the Tiger to the SnowLeopard directly, buying the OS from the Apple Store?

.- Do I need first to upgrade Tiger to Leopard? If so, how do I do that?

I know the very best option is buying a new Mac, but currently for me it doesn´t make any sense as I only want it to sync my devices...

So, any idea?

It´s been almost 10 days since I got my iPhone 4, and I cannot even start it!

Much appreciated

Regards

Javier Diaz

Spain