The SpacePak systems save space by using smaller ducts using higher velocity air. They are also fairly expensive systems (I think they get sold mainly to historic homes where installing regular ductwork is unacceptable and cost isn't an issue).
Many homeowners replace their radiators with forced air systems. The ductwork is usually run in the crawlspace, in the attic, and (in multi-story homes) closets often form a handy way to route them around. Sometimes they are put into a small soffit run along a hallway, and they are fairly unobtrusive. I lived in a 1930's vintage 3-story brick he that was retrofitted with ducts, and it was great. If you decided to go this route, you could install a "regular" gas forced-air furnace and a "regular" central AC system, (and put them in your garage if you want) which would save a lot on the cost of the equipment (vs a boiler), possibly offsetting most of th cost of installing the ducts. One caveat: If you put ducts in unconditioned space (e.g crawlspace or attic) be sure they are well insulated (R-13 as a minimum). This is a place installers try to save a buck, and it costs so little extra to do the job right and saves you $$ for decades.
Hydronic heating is another possibility. That could be done with a radiant floor as you suggested, which can be expensive (depending especially on how easy it is to access the underside of your floor) but results in a very comfortable home. If you don't want to spend that much money, you could put hydronic baseboard heaters in your rooms (see the Alpine home air web site for a picture and specs) and replace your existing galvanized pipes with modern PEX tubing (that is, the PEX would run in the same spots where your galvanized pipe is now, and the new baseboards would replace your radiators. The hydronic baseboards would probably have to be slightly longer than the width of your radiators, but that probably won't be a problem. They typically can transfer approx 500 BTU/HR per foot, so look at the BTU rating of your present boiler and divide by 500 for an estimate of the number of feet of baseboard radiators you'd need). Of course, with any hydronic system (baseboard or radiant floor) you wouldn't have any air ducts installed so you wouldn't be able to have central a/c.
As to whether you need a dedicated boiler or could get by with a good, high-efficiency water heater: What is the rating (in BTUs) of your present boiler, and is it approx the right size?