Mac Minis don't have dedicated graphics cards, so they'll always be a poor choice for higher end gaming.
Neither does the current 13" MBP(whether Classic or Retina). You need to move up to the 15" to get a dedicated GPU.
With that said, in 2004 the state of the art in graphics cards were cards like the Radeon X800 and nVidia GeForce 6800. The Mac X800 XT(retail) and GeForce 6800 Ultra(BTO for the G5) each had 256mb of VRAM, while some PC editions of these cards had 512mb.
GPU performance has increased exponentially in the last 10 years-the integrated Intel 3000 Graphics in my late 2011 MBP at least in most respects runs circles around the GeForce 6800 Ultra in one of my G5s. The 6800 Ultra is thicker than my MBP, and nearly as long as my MBP is wide.
Going even further back, I remember when Microsoft Train Simulator came out, and I
really wanted to play it on my desktop(rather than having to use my dad's). I think I had an AMD K6 running at 233mhz or so. I dropped a fair bit of money(at the time) for an AGP Radeon Rage Pro with 16mb of VRAM so that I could play it. Around that same time(from about 1999 with the B&W G3 to 2001 with the Digital Audio G4), Apple was cramming the Rage 128 into pretty much everything as the standard graphics card-and if you watch the keynote where the B&W G3 was introduced, this was touted as a selling point. Those of us who use still use PPC Macs on a regular basis regularly joke about how awful these cards are, although admittedly I have a huge stack of them(both in PCI and AGP, as well as some flashed PC AGP cards) for testing and diagnostic purposes as well as to just throw a working card in a rarely used computer.
I wouldn't worry about the graphics in any current Mac-integrated or otherwise-being insufficient to play a 10 year old game.
