I have noticed that, with my new 64-bit computer, more RAM is consumed just 'sitting' there, even with the older 32-bit software running (such as Word and Excel 2003).
Files that I was able to render on my old Win7Pro 32-bit system with 2GB of RAM using TurboCAD V15 were not able to render on my old system when I upgraded to V21 because I ran out of memory. This is most likely because V21 is larger than V15.
Another problem I had was that moderately simple files I created on my new system (Win7Pro 64-bit w/16 GB RAM) using V21 64-bit could not be rendered (or even opened, in some cases) with TC V21 32-bit. This was due to the 32-bit limitation on memory access.
On the plus side, I have had no noticeable problems running 16 year old software designed for WinXP on 64-bit Win7Pro, so I would be surprised if your "other older softwares" could not run on a 64-bit system. Specifically, UltraEdit V8, Protel 99SE (both from 1999), Office 2003 & Adobe Photoshop Elements 9 all run fine, although I did have to go into Administrator mode to register a couple of them.
So... I would suggest that you upgrade to a 64-bit system (Win7Pro, however, not Win8) and install all your older software; they should work fine.
a] 32-bit TurboCAD uses a different code base than 64-bit TurboCAD. I know this because: 1) there are different text strings used as warning results between the two versions; 2) for some objects, Selection Info will have a different description of the same object; 3) some functions will only work on the 32-bit version of TurboCAD or the 64-bit, but not both.
b] the default program that is tied to the .TCW suffix will be the last version you installed. So, if you plan on using 64-bit TurboCAD 2015 most commonly, install it last. I have 64-bit V21, 64-bit 2015 and 32-bit 2015, in that order and I was unable to re-assign the default to 64-bit 2015 even after editing the Registry.
TC Pro Platinum 2020, 2019, 2018, 2017, 2016 & 2015 (all with LightWorks & RedSDK) & V21
System: i7-5820K @ 3.30GHz, ASRock X99 Extreme4, 16GB DDR4-2133 RAM, Gigabyte GTX 970, Samsung NVMe SSD 950 (256GB), Windows 7 Pro (64-bit) SP1