I think that when you are putting together a "best gaming rig of 1989", you have to have a balance of forward and backward in mind. Before the 90s, I would agree with what KC9UDX said, "Nobody bought a PC to play games. They bought PCs for other reasons and played games because they had the PC", although having more advanced hardware inside a particular PC which was capable of playing the latest and greatest may have been a factor in an individual's purchasing decision.
When building a PC gaming rig, you should try to balance the machine's abilities to play games released before it as well as a PC for playing games after it. For 1989, a Tandy 1000 is a good choice looking back but not looking forward due to its lack of expandability and speed upgrades. A 486 was definitely in the breaking the bank category and unless its speed could be knocked down, very unfriendly toward older games.
I would suggest a happy compromise with a 386DX-25. Still pricey for 1989 without a doubt, but much less difficult than an early 486 machine. Slap in a compatible VGA card (one with a PVGA1A chip comes to mind), a Sound Blaster, 2MB of RAM and you should be good to go for gaming until 1993. If you want a high end system, include a Roland LA device.