I'm expecting #2 now, and she will be a little more than 3.5 years younger than my #1.
I wasn't ready for a second before now. It would have been more overwhelming for me than for my DS, but I think that's something important to know. Gauge your own capacity and energy.
I really appreciate that my older DS is in preschool now, much more independent, potty trained, sleeping through the night, etc. It will be a hard transition for him because he's gotten used to being an only up until now and because he's old enough to really "get" that things are changing. I've seen some younger kids adjust better because they just go with the flow a little more (but it depends on your kid a lot).
However, I am definitely feeling the gap between the baby years and now. I think a gap of about 3 years would be better than 3.5, though I know it's hard to plan it that exact. My DS is old enough that we haven't had to deal with real "baby" stuff in a while, and I'm starting to totally dread going back to that. I also realize that they will be 4 years apart in school (due to birthday timing more than actual years), and this will mean a longer period where one will be in daycare or at home with me (depending on what we decide to do). It also means just a longer period until they are older and independent. Just some things to consider.
Basically, I think anywhere from 2.5 to 3.5 is a pretty good number. Less than that is really hard, and more just extends the baby years for you (ugh!). I'd start trying with the aim of 2.5 or a little more and then see how long it takes to conceive. It took us about 6 months this time around.