Do I need Snow Leopard (10.6) to install Lion (10.7)?
Nope. There is a USB Flash Drive version of Lion that will install on any system without an operating system that meets the same requirements. For more limitations of Lion, see this tip:
Most important thing to remember Snow Leopard is the last Mac OS X to run PowerPC software on an Intel Mac natively. To tell if you have PowerPC software that won't run, I've made the following tip:
You can run Snow Leopard Server on 10.7 or later through this tip.
10.5.8 is the most current operating system if you have a PowerPC G4 867 Mhz (G4 800 Mhz dual processor can't natively support 10.5 or later)
or 1 Ghz or faster, or PowerPC G5. 10.5 does not support Classic, but does support dual booting into Mac OS 9.
10.5 is the minimum for Intel Macs to support Boot Camp, the Windows virtualization engine.
10.6 requirements should be read if you want to go the software path to upgrade to Lion or Mountain Lion.
10.6.6 is the minimum required to download Lion from the Apple Mac App Store.
10.6.8 is the minimum required to download Mountain Lion (10.8) or Mavericks (10.9) from the Apple Mac App Store.
Mountain Lion limitations are described here.
No G5 is an Intel, and no Intel is a G5.