On my single 867, Leopard is slightly slower, but not greatly so. That said, the extra features (Time Machine, Spaces, etc) easily justify the slight speed drop. Furthermore, it's much easier to get current or nearly current versions of applications for Leopard than Tiger (in large part due to the Leopard/Snow Leopard APIs that don't exist in Tiger, but also just because any developer still thinking PPC is most likely to just worry about the newest version...).
Your machine is plenty quick enough to run Leopard nicely, especially given that you've maxed out the RAM. The one downside to Leopard is that when you don't have a Core Image capable graphics card (unlikely, given that the only one that works in your machine is the aftermarket Radeon 9800), it software-renders Core Image effects (using CPUs+system RAM) instead of ignoring them altogether as Tiger does. Frankly, I'm given to believe this is the main reason most older Mac users feel that Leopard is slower than Tiger--but the effect will be minimized for you, given your dual processors and (relatively) high amount of RAM.