--- Document valid as of end of 2010; updates are expected...

Recent versions of AIX require IBM RISC (RS/6000, pSeries, System p, POWER, Power Systems) hardware.

Should you need more info, or have corrections, additions, feel free to contact me on IRC, or drop some comments below. oops, sorry, this won't work here ;)

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OS levels, software, license

Which level of AIX should I install?
As the OS level decides which machines are suitable running it, this should be your first decision.
It is worth having a supported level of the OS which still receives security fixes. As of mid-2010, 5.2 and older is unsupported now. 5.3 is getting to the end of its lifecycle but it is quite mature and stable now after 12 technology levels (these are the major fix/patch levels in AIX). 6.1 offers a lot of additional benefits (enhanced security, WPARs, better RAS capabilities), so I recommend running 6.1 if your budget allows it.

Does AIX have an activation key or serial number?
No.

What can I download from IBM for AIX without any entitlement?
OS updates within the AIX major release, betas, extras (Expansion Pack, AIX Toolbox).

What about the AIX license?
The license is bound to the support contract. During the post-install phase, you must accept both the software license agreement and the support conditions. The license is transferrable while it has not expired, the transfer implies an agreement to the license terms by the new owner.

Can I buy IBM support for my machine bought from a third party?
Yes. It costs a lot, though.

Hardware models, compatibility, buying

Which models are capable of running which OS levels?
Generally speaking, AIX 6.1 and the forthcoming 7.1 supports POWER4 and newer models. AIX 5.3 runs on some of the PowerPC 604 equipped models - check the AIX 5.3 Release Notes for supported models. For 5.3, you need the newer PCI architecture which is called CHRP by IBM.

Which are some recommended models I should look for?
For 5.3, the 7044-170 (44p Model 170) is a popular choice with its 333~400MHz POWER3 64bit CPU and standard LVD SCSI disks. For 6.1, the cheapest choice is the 9114-275 (Intellistation 275) with a 1GHz POWER4 CPU, but it requires special hotplug disk trays which are hard to find. Both are too loud for daily operation in a regular home (there are at least four fans in each).
POWER5 models are still expensive. The Intellistation 285 or the server grade 505, 510 and 520 are in a lower price range, see below.

Could you give estimate prices for used models?
In Europe, the supply is quite good and the prices are fair, you can get a 150, a B50 or a 170 for 50 EUR, a POWER4 275 for 100 EUR (sometimes without disks). The Intellistation POWER5 285 starts from 400 EUR. The cheapest POWER6 box on eBay is currently somewhat under 8000 USD.

Are there machines other than IBM RISC capable of running AIX?
Some older Bull PowerPC workstations and servers (for example the Escala line) are suitable, and the old Apple Network Server as well. In general, no. Especially not Apple ppc models.

May I use 3rd party parts (RAM, HDD, VGA, Ethernet...)?
Almost certainly no, except harddisks. In most cases, you cannot even swap RAM between models of the same class. Try to buy the machine with a big initial setup. If you buy spares, either ask the seller to guarantee that they come from the same model, or make sure the FRU numbers match your model's part list. Foreign PCI cards are not supported by AIX drivers because of the firmware signature etc. The only exception is the Matrox G450 PCI video card which is identical to the IBM GXT135p. Forget about using SATA controllers/disks, USB controllers as well. Note: you can use USB mass storage devices to a limited extent on POWER5 and above models having a factory USB 2.0 port. Other parts (PSU, fans etc) are also custom and hard to find, so be prepared for paying a nice sum in case of a hardware failure.

After purchase

What do I do if I have no install media?
Just to mention, some AIX images are available on torrent sites, but apart from the legal issues, who would trust in it, espacially without at least an official checksum? If you are an IBM AIX customer, you are entitled to download media for the OS levels you have paid support for. Sometimes you may find install kits for sale online.

What should I do if I want to preserve the original installation?
Read the alt_disk_install manual or get familiar with the mksysb utility. On newer releases, you can create a bootable CD/DVD backup with it.

I have a used machine with the original OS installed. How can I gain root access?
If you are lucky, you can find an active guest account or a working remote hole. A more successful attempt is to get a copy of the first install CD/DVD and do a service mode boot (equal to single user). Sometimes you need the exact fix level which may make things more complicated. Unfortunately, another rootvg disk cannot be imported (thus mounted) from the running OS.

How do I check hardware resources in a machine?
You can use the prtconf command from the OS or use the boot and the SMS menus from the firmware, and look for the information entries.

What are the symptoms of a hardware error?
If you have a newer model, the yellow LED on the front panel may be on, but it can mean a simple power loss. If the machine stops booting and a code is shown on the LED panel, check for it in the diagnostics guide.

Alternative access (if you don't need a machine of your own)

Is there an emulator for running AIX?
A commercial product, Virtutech's Simics is a hardware modeling emulator which can be extended with further CPU libraries including POWER, these are not included in the default installation and must be purchased separately. 

AIX vs GNU/Linux