Radiant floors are "cruisin' in yer socks" cushy, and can usually operate as lower temperatures than old-skool radiators. For every 10F that you can lower the water temp you reap about a ~3% savings in fuel use. If you can heat the place using the floors as radiators with 120F water instead of 180F, that's a 15-20% savings.
If you're doing just one room with low-temp radiation and still need high temp everywhere else it makes for a more complicated system design, but it can be done. If it can all be done with low temp water, further big savings can be gotten out of a condensing boiler. Running one small zone of "staple up" retrofit radiant with low temp water usually requires a buffer tank to keep the boiler from short-cycling and losing efficiency, but properly buffered it's more efficient than high temp hydronic.
Retrofitting radiant floor is expensive, and won't pay back in fuel savings alone, but in creature-comfort it does big-time. It may never pay back ever on fuel savings alone, but radiant floors are nice, and a value adder at resale time.
It's a big subject- hire a pro who stands by their work, and understands your efficiency, cost, & comfort goals.