Well first off, greetings as I havent been around these parts in a while;
anyways...

I was looking into getting an old powerpc mac for testing and whatnot, and I figured it would be cool if I could get some old betas of Mac OS X running on it, specifically the 4 developer previews and the Public Beta as well as possibly the PowerPC rhapsody previews and OSX Server 10.0 with the platinum interface.
Problem is, a quick google search failed to reveal what model of old Mac would have the best compatibility with those.

So, does anybody know which old mac systems would be best to run some of the aforementioned OSes on?

PowerMac G3 (or even G4) sound quite fitting for the task.

Hmm, the g3 does seem like it would be alright for that purpose; although im inclined to think theg4 might be maybe too new for some of the betas, although I did read that the PMG3s cpu could be upgraded to a g4...
although I am just guessin, i really know jack about powerpc macs

There is no perfect system, for OS 0-7.6 a SE/30 would be an ideal system as it's fast enough to run 7.6 no problems, but doesn't need system enablers.

For OS 8-9.1 a PowerPC 603 or 604 system is best, as they too are fast enough to run at a decent speed (with enough ram, 32 megs or more) but don't require enablers.

For Mac OS 9.2-10.2 Betas a Blue & White G3 with stock Rage 128 card is great. This system remains compatibility for most OSes.

For 10.3-10.5 a G4 tower, 450 MHz+ with a Rage 128 card is a good choice. While the G3 can technically run up to 10.4, it feels sluggish to me, while the Powermac G4 AGP Graphics I have feels a lot faster.

Now, one thing is these machines are being stuck right into the middle of their support, such as the SE/30 can run upto 8.1. I choose these sections of models for compatibility and speed reasons, I honestly don't think a 30 MHz '030 is going to be very fast at all running 8.1 for anything. PowerPC 603s can run 7.5, but with a PowerPC enabler which might cause problems. The G3 can run 8.5-10.4, but for compatibility reasons a 603 is a better choice to me as many programs from that era took advantage of them. The G4 runs OS9-10.4 as well, but being as a G3 can pick up the backend it works a bit better.

my G4 mirror door tower with 450Mhz always ran well with 10.4. My daughter's g3 tower runs 10.3 like a champ, had to max the ram out but, hey, it works, right?

For Mac OS X Developer Preview and Public Beta, Use a Mac from the correct time period, an earlier PowerMac G4 (Yikes or Sawtooth) will run either one of them, the later Gigabit Ethernet Model may have some issues since that's a newer model, Anything later than that (Quicksilver) will not work

the best bet would be the B&W G3 tower... when OS X Server 1.0 came out (with discs stamped 1.1 there was plenty of confusion!) I had one of the 333Mhz G3 iMacs and all was well there. I then wanted a tower so I later got the G4 AGP sawtooth and found out that OS X 1.x wouldn't run until the 1.2v3 edition which mine wasnt bootable....

At least the B&W G3 is easily upgradeable hard disk and memory wise. ..

The OS X 10.0 preview and onwards ran on the G4, which I did manage to get at the time.. Oni for OS X was 'fun' back then...

You need at least a PPC 604 or 603 for earlier builds.
Remember Rhapsody DR2 has weird memory limits and 10.5 can run on old macs if you adjust OF to tell it has more than 867 MHZ.
OS X Server 1.2.1 was for G4 Gigabit Ethernet.
QEMU can emulate a PPC Mac if you use OpenHackWare, but I haven't tested it yet.
I have a blue & white G3 and a G4 DA but you are living too far away to sell.

Early iMac G3's are useful for OS X betas aswell.

For me, the best choice is a PowerMac G3 B&W : he can run all the old bêta, you can change/overclock the CPU, change memory, hard disk (i try with a PATA SSD) and graphic card.

Just verify one thing : the earlier version of the G3 B&W has a problem with IDE controller, and it's problematic with "modern" HDD.

I try Mac OS X DP3 and 4 on one of my G3, with no problem.

My G3 has a CMD controller that is buggy but AFAIK only with larger disks than the builtin.The Linux driver has a lot of code added to work around the bugs.
Its easy to install (SCSI PATA SATA) PCI controller cards or connect a Compact Flash card adaptor to the much slower CD connector.Another problem is that these machines can't boot from every DVD drive and booting from USB or Firewire is officially not supported but possible via OpenFirmware prompt.

I'm lucky : i use a DVD writer from Samsung (SH-202) and it works directly.

So a bit of an update I guess,
Doin a bit of research and pluggin around in NeXTSTEP & Rhapsody Dr2 got me really into running Mac OS X Server 1.x in the near future...
I been looking around ebay and whatnot for a decent G3/early g4 rig to set up Server 1.x on; now I found two PowerMacs at a decent price range, so Im thinking of grabbin one of the two... now one is a g3 (blue&white) and the other is from the first round of G4S made.
Lookin around online, I've read that one needs Server 1.2 or 1.2v3 for a G4 machine, and that some people have reported (including louisw3 above) that their disks are not bootable...
So I'm wondering, is that the case for the disks on the ftp and if they are not bootable, what I would need to do to get em running on the G4.

Edit: so I got an email from someone who could sell me a PowerMac 9600/300mhz, which upon research should run OS X Server 1.x, however I have not been abe to find out of Mac OS X DP will run on it OK, so if anyone knows could they let me know?

Mac OS X doesn't officially support Pre-G3 Macs, But you can use 3rd-party workarounds (XPostFacto) to get OS X working on those older macs, Not sure if that would work on Mac OS X DP or Public Beta, But it does work on OS X 10.2.8 or earlier (10.4.11 if the CPU was upgraded to a G3), Should work if the machine has 128MB of RAM or more