As the Quad PowerMac G5's start to arrive, more benchmarks are emerging.
Barefeats posts a collection of benchmarks from After Effects, Cinebench G5, Photoshop CS2, Xbench, and UT2004 comparing the Dual Dual-Core ("Quad") G5 2.5GHz PowerMac to the previous Dual 2.5GHz G5 PowerMac.
Meanwhile, Macsonly also provides the same comparison with some similar benchmarks but also some "Real World" tests.