If you play Battlefield the Dual 867 will be faster mostly because you can use a Video Card much better than what the mini has. Also I believe BF has some SMP optimizations.
Applications don't really need to be optimized to see a boost on a DP system, you will notice this if you multitask alot and even if you don't now you will learn how to given the headroom. I can have like 6 windows in Safari open, Burn a DVD, Listen to itunes, iphoto open, mail open, Limewire running, Working with Logic Pro, etc etc all at the same time without so much of a hiccup on my Dual 2.5 sure not all these apps are SMP aware but they are all running on their own threads for each respective functions and OSX utilizes all functions across both CPU's
Just for your note the Dual 867 had 256k of on chip cache and 1MB of L3 per processor and ran on a 133Mhz FSB compared to mac mini @ 1.25Ghz w512k on chip Cache.
Ignore Xbench scores they are utterly useless, Xbench does not account for MP and is inaccurate at benchmarking the Video Card. With a decent Video Card and faster HD than the mini the PowerMac will feel more responsive in most tasks.