Yesterday, I posted about some ideas for upgrades to the Tandy 1000 SX that I have on its way. The two most obvious ones are the NEC V20 CPU and the 8087 math co-processor.
Back in the day, Intel's 80x86 CPU line did not include a built in math co-processor. You had to purchase and install one separately all the way up through the 486DX if you wanted floating-point operations. Not a lot of software took advantage of this, and I never installed one back in the day. Even my Pentium-class machine was a Cyrix 686, which was slower than Intel's CPUs at floating point but faster in other areas. But it's something I had always wanted in my Tandy 1000 SX back then, and they're fairly inexpensive on eBay, so why not?
The other upgrade, which I hadn't done to my Tandy but had done to another, generic, XT-compatible machine, that I plan on is to swap the processor with an NEC V20. It is pin-compatible with the Intel 8088 but will give you a little more speed at the same clock rate. (The XT-class machine I had one in also had a crystal hack done, which also provided a little bit of speed boost.)