Incase there was any doubt over the mac mini vs G5, theres this benchmark of an old core duo imac vs a G5:
http://www.barefeats.com/imcd.html
Thats a Core Duo, not a Core 2 Duo (which is significantly better), and the imac stomps all over the poor G5. A Mac Mini with a Core 2 Duo would be even faster. G5's really are a rip off when you can get stuff that is so much faster for so much less.
And heres some game benchmarks for the GMA 950
http://www.anandtech.com/video/showdoc.aspx?i=2427&p=2
It will be able to handle Everquest easy enough. And remember that you can use the Windows version with an intel mac thanks to bootcamp. It could play Everquest 2 fine, since that came out when UT2004 did, the GMA can play UT2k4 fine at the highest settings according to those benchmarks, and that game was a powerhouse back then. The first Everquest will be a breeze.