Im pretty sure the gaming performance increase for getting a 2500+ Barton over a 1700+ is significant (+ a faster fsb??!)
Additional cache has a small impact on gaming, once you have enough of it.
AXP 2700+ (256KB cache) vs Athlon XP Barton 3000+( 512KB cache, same clock speed )
UT2003 flyby: Barton 8% faster 215fps vs 199fps
botmatch: 8% faster 75fps vs 70fps
Jedi Knight 2: Barton 4.5% faster 168fps vs 161 fps
Comanche 4: Barton 3.7% faster
I doubt anyone could notice any of the above differences while gaming.
The faster FSB can easily be neutralized by overclocking, and that's probably why the Barton scores higher than the TBred, because of the FSB, not the cache.