You can add VGA only cards, or VGA/EGA cards.
Some background on the available monitors for Tandy's:
The Tandy came with the possibility using a CM-5 or CM-11 monitor (there may have been others, but these were the main ones). The CM-11 had a lower dot pitch and therefore a sharper image. There was also a few extra resolutions for software that supported it (I believe mainly spreadsheet programs might have supported this, but I haven't looked extensively). The CM-5 was a more common monitor, as it was cheaper, and for most was sufficient for the time.
In an effort to remain authentic, I would suggest you get a VGA/EGA combo card. You can then use VGA with a modern monitor, but if you come across a Tandy monitor or an EGA monitor, you can hook that up for a more 'authentic' experience. Most VGA/EGA cards can support Tandy monitors, and there were even some programs that supported 16 color EGA, but not Tandy 16 color. Most all 16 color games did support EGA though, and Tandy monitors can display EGA.
This is probably more information than you need, but I think going with a VGA only card will limit you, and if you find a Tandy monitor, you will end up having to remove the card again just to use the older style tandy monitors (and waste the money you spent on a vga card).