I have a 5-year-old, a 4-year-old, and an 18-month-old. My older two are 17 months apart, so I was exactly where you are about 4 1/2 years ago. :-)
We decided not to move my oldest out of the crib or her room at all before the new baby was born, and the new baby slept in a bassinet in our room. When my oldest was about 19 months old, we started to move her into a twin bed in the same room as her crib. Honestly, we had no idea what we were going to do with the new baby yet, but we had to consider our older child's needs first. We started with just naps at first, and once that was going well, she started sleeping in the twin bed at night. I'd say it took about a month to complete the transition.
Another month later (so when my oldest was 21 months old), she asked to sleep in our guest room that had a double bed in it. I jumped right on it and moved her over there that day. I pushed the bed against the wall, put the bedrail up, etc. I think because it was her idea and we acted so quickly, it went well. A couple weeks later, we moved the new baby into the crib. (Okay, actually, the new baby mostly co-slept with us from the beginning anyway, so we weren't in desperate need of freeing up the crib, but it was nice to have the option. ;-) )
One thing you might want to try that I wish we'd tried is to play some white noise in your son's room to help drown out any noise from the new baby. Start it now, and he'll already be used to it when the baby comes. My girls have a white noise machine (or they listen to a classical music radio station) when they sleep, and it definitely helps.
Just take things one day at a time, and you'll find something that works for your family. Good luck with the new baby!