The most common engine swap is probably to fit your volvo with the B21ET engine, together with an A-camshaft an 940-intercooler and higher boost, this engine will quiet easy and ofcourse cheap produce above 200hp. In sweden this is also the easiest combination which is accepted by test centers. (I´m not sure what opposit in the US is called..)
But, if you are in fore more power, I know those who have upgraded their 240 with 940 turbo engines. if you´re the owner of a 240 equipped with a carburator, it is much harder to put in any turbo engine, than it is with an injection engine. The 1990 volvo 240 ( with B230F engine ) has got almost the same electrical system as the Turbocharged B230FK or B230FT (witch are actually the same engine..) and a swap would probably be pretty easy. But you vill need ALL the elctrical stuff from the 940, and probably a few days to get it straigtend out.
These engines are mostly equipped with an m90 gearbox, that is a lot stronger then the original m47. Older turboengines as the b23ET came with an m46 gearbox, that is also found in the 240. This installation is easier, with the m90, you vill need an hydraulic clutch (not found in swedish 240:s, i´m not sure about US..) and some violence to the tunnel where it is supposed to fit. I´d say this is the hardest part, in changing your engine.
sum up: if you are the owner of a 1990 model it is quiet easy to fit the engine istself, but slightly harder to get the gearbox working. if you want to use your old gearbox, I think you will have to do some welding to make the clutch fit. My tip is to get your hand on an older 240 turbo engine (b21Et) with T16 turbo and an extra injector you can get all the power you need.
About the five cylinder engine, sure I´ve seen a few in magazines but it is not very common, You will need to rebuild most of your car to make it work.
Here in sweden the biggest problem is to get it through the Car test once a year, they don´t like when you do things to your engine, so most things are forbidden by law to to, mostly because of emmission law and stuff..
I don´t know if I answered your question, but If there´s something specific you´d like to know, I´d be happy trying to solve it!