UT2003, Q3, X²
We've purposely chosen a 'bot match from the retail game. It's run at 800x600 to minimise the graphics limitations that even a Radeon 9800 Pro may introduce. Bot matches are characterised by the need for excellent subsystem performance, performance which first emanates from the CPU. The Extreme Edition manages to add just under 10% to the regular 3.2GHz's benchmark. Assuming that we could fully unlock the latter CPU and run memory at the specified DDR400 speeds, we hazard that a clock speed of 3.6GHz+ would be in order to compete with Extreme Edition's 3.2GHz. The extra cache is not a gimmick. It does have real-world performance benefits.
X²: The Threat usually has returns a close set of results for high-end CPUs. Excepting the Barton XP3200+ and the high performance of the Athlon 64 3200+, we see that the E.E does allow for a little extra oomph. The benchmark is still scaling at 3.6GHz.
If ever there was a vindication or justification for the extra 110 million transistors that make up the 2MB of L3 cache Quake III is probably it. The Extreme Edition adds another whopping 21.8% to the regular 3.2's scores. That's largely down to the lack of L3 latency when compared to system memory.
