I'm pushing my PowerMac G5 as far as I can before replacing it with a Mac Pro (probably the next upcoming beast of 2010).
I have to say it's still really up to date with today's computing needs. I'm a really heavy user (Adobe CS4 mainly), and PM holds it very well so far.
As you all know, problem is the end of the road came with Snow Leopard.
Anyway, here is how I doped out my machine to handle OS X Leo with most current apps
Dual 2.3GHz (late 2005)
16 Go RAM
1 To hard drive RAID 1 (2x500 Go WD Caviar)
Geforce 7800 GTX (PCI-E)
I still holds up against very well against some Intel machines.
Thus, I don't think it's retro as of yet, but any Mac pre Intel is more or less retro... Isn'it ?
@Peter : I start to see some limitations now with my PowerMac G5 (2x2.3GHz, 8Go RAM, Geforce 7800 GTX 512, 1To HDD). I could care less about Snow Leopard, but the film editing part of Adobe CS4 can't be installed on PPC. Same thing with Google Chrome browser, and mediacenter software Plex. Those are just examples, but it is definitely the end of the road for PPC.
Very upsetting because as far as performance goes, my PowerMac reads HD movies 1080p, and any demanding task I request. It beats my MacBook Air and my Mac Mini Intel 1.6GHz. I have no choice, but to upgrade because my work (Internet consultant) demands to be on top of technology.
Anyway, get at least the 2.23 or 2.7, but Quad is a beast. I do not recommend the 1.8 at all.
I agree with LaurentB, for myself 'retro' ends with 9.2.2 capable Macs as the G4. Updating them to 10.4 is acceptable, and if there is anyone outside doing this to hers/his iMac G3 those will have really slow systems.
But as time goes on, there are two criteria for what a 'RetroMac' is: Apple support or the capability to run 'Classic' applications.