As your systems are quite old, the best GPUs you can put in, appearing to be the GTX 750ti, will be woefully underpowered in a new PC, even a basic one. If you do plan to build a newer PC at some point in the near future, it might be best to start working at that instead. At least for most people, the kind of power they would hope to get out of a new PC would require another new GPU, which you seem to already be planning. Unless you are thinking about waiting over a year before getting a new PC, I would just wait and skip putting a GPU in the old system.
While a GTX 750ti will work in a new PC just fine, it wont perform as the new computer should. Even the cheapest, most basic, new GPU will perform over twice as well.
Go for the 750ti if your power supply can handle it, and you plan to keep and use this computer for a while to come, otherwise it might just be a waste of money to upgrade it now.