Whether or not you can upgrade to 10.6 depends on what Mac model you have. These are the system requirements for Snow Leopard:
Mac computer with an Intel processor
• 1GB of memory (I'd have 2GB at least)
• 5GB of available disk space (make that 30GB at least)
• DVD drive for installation
* Some features require a compatible Internet service provider; fees may apply.
* Some features require Apple’s MobileMe service; fees and terms apply.
If your MacBook Pro meets those requirements, all you have to do is buy a Snow Leopard disk for $29 and you can go directly to 10.6. However, if you have any PowerPC applications that run on Mac OS 9.x.x, 10.6 has no support for the Classic environment, although applications themselves should run using Rosetta. But if you upgrade later to Lion or beyond, there is no support for anything to run a PowerPC application.
Likewise, if you have applications that run under 10.4.11 now, upgrading the OS will almost certainly require updating (buying) a new, compatible version for 10.6, which could be very expensive, so investigate the costs of that before you upgrade anything. iMovie 9 was part of iLife '09, which is no longer packaged with new Macs or sold; we're now at iLife '11, which would have a later version of iMovie included. You'd have to buy iLife '11 for $49 from the online Apple Store, or from your nearest Apple Retail Store.