Okay, so background to my answer: (1) I use PowerPC and Intel Macs (Mac User since '94) and (2) I have been using DAW Systems since 2000, from ProTools 5 and Logic I believe 5 or 6, not sure, but anyway, a Long Time.
My recommendation from an AUDIO PRODUCTION point of view:
Get the Intel Mac Mini. While it, under certain conditions, under certain circumstances could potentially be slightly less fast than a PowerMac G5 Quad using a specific G5 optimized rendering application, it is easily the better choice both for DAWs and looking forward. PowerPC Based Macs are no longer suitable to have as your main Recording Mac if your just starting out on the Mac, or only have limited external hardware (ie USB/FW Based Interfaces) (obviously if youve been recording on the Mac since pre-history and have large amounts of random audio hardware on shelves like me, my advice differs, as then you have older project files and lots of other specialised PPC tools kicking around - If you are even considering an Intel mac, this cannot be the case, as you would only consider PPC Macs if you had PPC only-never-to-be-updated-tools, or only a Mac Pro if you required Internal Expansion Cards for your interfaces).
Firstly, graphics are not in anyway important to the issue here, so we can safely ignore that argument, as both systems can happily drive a 30" Display, no bother, and if your considering a Mac Mini, you dont need expansion, as if you did, you would never even consider a Mini. (Also, the Minis RAM is very easy to upgrade on the new design, you just flip it over and open the door and Boom, there it is - much better now than in the old ones, so that argument is moot.) Also, the amount of RAM is not really going to be an issue unless your looking at 100+ Tracks, and if that was the case, you again wouldnt be looking at a Mini under any circumstances (Or a G5 Quad - Neither CPU would be able to keep up). 8GB will be fine. My Main Audio Production Mac only has 12GB, and I rarely go above 8 unless Im video editing, never had it go above 8 doing Audio Work (Even with all 4 DAWs up and Running, and Soundtrack Pro and Waveburner together).
Secondly, Intel Macs are the best looking forward, because as your experience with the Mac (and Audio in General) grows (we're all learning), you'll acquire more and more tools to let you do more things (I started with just ProTools on a PowerMac, now I have Logic Pro, ProTools, Digital Performer and Reason all in use as each does somethings well, and others less so), and as time progresses finding the PowerPC Versions of these tools is becoming harder and harder. (I had to acquire some Logic Pro 7 Discs for a client recently, it took me a Month to find a source that could supply me the application disks, serial, XSKey and the documentation for a reasonable price, its a similar situation for older versions of ProTools). If you choose to follow my advice and go the Intel route, upgrade to Logic Pro 9, as its much faster and more compatible on Intel Machines (I tested Logic Pro 8 on my MacBook, and a G5 could equal it despite the G5 being Dual 1.8, and the MacBook Dual 2.1, with Logic Pro 9 the MacBook is about twice as fast as the G5), and having the latest version of Logic always helps when it comes to working out how to get plugin X to work with it. The last advantage of an Intel Mac Mini is that it lets you run the latest version of Sibelius, which again is a massive bonus over the G5 (Same reasoning as for Logic Pro 9).