Install Mac OS X 10.6 from a DVD; this is free for paid MobileMe members and available in the online Apple Store for other people. If you go to Lion, you won't be able to use Office 2004 but can use Office 2008 or newer or alternative products such as OpenOffice.
Unless you use the Lion thumbdrive, you'll need to go through Mac OS X 10.6 anyway.
Snow Leopard/10.6.x Requirements...
* Mac computer with an Intel processor
* 1GB of memory (I say 4GB at least)
* 5GB of available disk space (I say 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.
Which apps work with Mac OS X 10.6?...
It looks like they might still have it...
If it's a core Duo & not a Core2Duo, then it'll only run in 32 bit mode.
Lion/101.7 System requirements
• x86-64 processor (Macs with an Intel Core 2 Duo, Intel Core i3, Intel Core i5, Intel Core i7, or Xeon processor.)
• At least 2GB of memory
• Latest version of Mac OS X Snow Leopard (10.6.8), with the Mac App Store installed
• At least 4GB of disk space for downloading
Like Snow Leopard, Lion does not support PowerPC-based Macs (e.g., Power Macs, PowerBooks, iBooks, iMacs (G3-G5), eMacs).
Lion also does not support 32-bit Intel Core Duo or Core Solo based Macs. Rosetta is no longer available in Lion, which means Lion no longer supports PowerPC applications.
What applications are not compatible with Mac OS X 10.7 "Lion"?
If you are going to update MS Office 2008 instead of Office 2011, be sure to install it in Snow Leopard BEFORE you upgrade it to Lion: its installer has some PPC code and needs the Rosetta in Snow Leopard to install. The programs will run in Lion. Office 2011 solves this installation problem and can be installed and operated in Lion.