All true Tandy 1000s (true Tandys do not include any RLX or RSX machines) come shipped from the factory with variable amounts of RAM, and all can be upgraded. Here is the basic chart :
128K - Tandy 1000/A
256K - Tandy 1000HD, 1000EX, 1000SX (also Tandy 1000/A)
384K - Tandy 1000SX, 1000SL (also Tandy 1000/A/HD, 1000EX, 1000HX)
512K - Tandy 1000SL/2, 1000RL/RL-HD (also Tandy 1000/A)
640K - Tandy 1000TX, 1000TL, 1000TL/2, 1000TL/3 (all other upgraded Tandys)
768K - Tandy 1000RL/RL-HD, 1000TX, 1000TL, 1000TL/2, 1000TL/3 (upgraded)
Now you might think that more RAM is always better, but consider a few of these games :
King's Quest, Touchdown Football - The PCjr. Only Booters -versions of these games assume that the video memory is at the top or within the first 128KB of RAM, as it would be on a PCjr. This assumption is also true for the Tandy 1000/A, but vanishes when you upgrade the RAM with an expansion card. The Tandy 1000 HD comes with one of these cards pre-installed but can be removed to bring the system back down to 128KB. The hard drive controller card that comes with the system also uses DMA, so it may have to be removed. The Tandy versions of these games work fine so long as the RAM is 640KB or Less
Demon Attack - The Tandy version of this game, which unlike the PCjr. version came on disk, will work with RAM up to 640KB, but will fail to work if 768KB is in the system. This was well before any idea was given to 768KB. Touchdown Football also has the same issue, it will not work correctly if 768KB is installed.
Demon's Forge - The Mastertonic 1987 PC port of this game supports 16-color Tandy graphics only if there is no DMA chip in the system. Otherwise it supports 4-color CGA graphics. The Tandy 1000/A, EX and HX all do not come with DMA, but to upgrade the RAM, you must use a board with a DMA chip included. I believe this may be a hold over from its port to the PCjr.
Demon Stalkers, Duck Tales : Quest for the Gold, The Simpsons : Arcade and Snow Strike - These late 1980s games will show no graphics, garbled graphics or fail to work if the RAM has been upgraded beyond 640KB. Apparently these games were not programmed with the idea that you could expand a Tandy beyond 640KB, despite the TX already being released by this time. To play these games, you need to remove the RAM upgrades in a system with 768KB. All require 512KB, so apparently only two possibilities were envisioned for these games. Alternatively, you can use their CGA command line arguments (C, CGA, C, respectively) to play the games with CGA graphics and Tandy sound.