Command & Conquer: Generals
With my old Dual 1.0 GHz "MDD" G4 (1.5GM RAM, Radeon 9000 Pro) I had to play at 1024x768 with the lowest quality settings, and even then it was jumpy at times and NEVER perfectly smooth. With my new Dual 2.5GHz G5 (2.0GB RAM, Radeon 9800XT), I can play at 1600x1200 with the highest quality settings, and its only jump occasionally on HUGE third-party maps in the late game (when there are tons of units moving around). It is buttery smooth 99% of the time!
Unreal Tournament 2004
With my old G4 I had to play at 800x600 with the lowest quality settings, and even then it was pretty slow and jerky a LOT. With my new G5, I can play at 1600x1200 with the highest quality settings for ALL options, and its been perfectly smooth on every map I've played so far (maybe 3-4, since I don't play FPS games a lot).
Older games (like Neverwinter Nights and Quake III) were usually fine on the G4, but still usually needed to be run with at least a few settings turned down. Now, as expected, I can run them at 1600x1200 with all settings maxed and there is never any sort of slowdown.
I'm going to install Return to Castle Wolfenstein tonight, but I expect even better results than UT2004 since RtCW must have much lower system requirements.
If anyone tells me how to benchmark these games, I'll try and do so in the next day or so.