You're mini probably has the same graphics card as my macbook. nvidia 9400m? While its an adequate card for general use and a bit of gaming, I'm running race07 mainly on mine (via bootcamp), Its not really beefy enough for high end graphics.
You're options are very limited if you want to keep using you're mac for gaming, upgrade the ram to the maximum and make sure you have plenty of free HD space for use as vram. Thats about it I'm afraid as there is no way to upgrade the graphics card and that really is the limiting factor here...
Macs are great and I love mine too but for serious gaming performance you will have to get a PC. You can pick up a decent spec gaming PC for a few hundred pounds these days so maybe get one for gaming and use you're mac for everything else....thats my long term plan anyway....