In what ways, specifically, did the G5 outperform the mini? I don't think anybody actually said that things which were completely disk-bound would be better on a mini, although I do remember saying that 2.5-inch 5400rpm disks aren't bad, as I've been using them in my main computer and haven't felt the need or desire to upgrade to faster ones. If disk performance is absolutely the limiting factor on your use of anything other than a G5, then the only upgrade I can recommend is either switching away from a Mac where you can get an inexpensive, low power consumption machine that can hold 2 3.5-inch SATA disks, or a Mac Pro.
That having been said, I envy what must be the prodigious amounts of spare time you've got , because I would never have found the time to switch my main computer not just once but two or three times in the span of a year. I'd gotten a new PC laptop in January 2009, and it has been my main (almost my only) computer ever since. The only thing I've really done to disturb the machine's continuous availability was to upgrade to Windows 7 -- which was a process that took me nearly a month in terms of planning so I could get it done as quickly as possible.
Owning a recent computer does not qualify as LAGS. It would qualify as such if, with said computer, you were indeed compelled to continue upgrading it or replacing it for whatever reason -- which seems to be what happened, admittedly.
While the mini isn't as visually robust as a PowerMac G5, I have a hard time thinking of ways in which it is actually
less capable at anything. The HDMI input/output adapter from blackmagic design (
http://www.blackmagic-design.com/products/intensity/ ) is one legitimate need for a Machine with slots, but PowerPC systems aren't supported, "yet" which means it's unlikely to be. However, the blackmagic design USB video input adapter (
http://www.blackmagic-design.com/products/videorecorder/installation/ ) doesn't specify whether or not it requires PowerPC, but it's also likely to work very well with a Mac mini or iMac.
(Note: There are a few other more expensive cards that do work with the G5 -- but if you do any work that would require one of these cards, you probably either already own one, or are paid well enough to be able to justify the purchase price of a Mac Pro in order to take advantage of the additional speed and capability so you can do more of this work so you can get paid more.)
But like I said, you never specified what type of work you are/were doing with the machine.
And so, for me, $460 would go into a new 2009 Mac Mini.