Short answer: yes, a 3/4 HP dust collector hooked directly to a bandsaw or small jointer can be adequate, but there are several factors involved.
Horsepower is a very rough estimation of a machine's capability. Impeller size, inlet size, filter efficiency (which often involves considering type and surface area), and real-world vs. advertised specs are also important factors.
If you want to know whether a given dust collector will be adequate for a given machine, you should start by looking at two numbers: CFM and static pressure.
For reference, one 3/4 HP dust collector is advertised as pulling 650 CFM, but its static pressure is not listed in the specs, and the advertised specs are often exaggerated. Even though a dust collector may advertise 650 CFM, it may only deliver 500 CFM directly at the dust port. Adding 10 feet of flex hose may degrade the performance below 350 CFM.
Various types of tools have known baseline requirements for adequate dust collection, measured in CFM (cubic feet per minute). If the dust collector does not meet this requirement (or if it barely meets it), move on, because you'll never be satisfied with its performance.
Static pressure becomes important when you start factoring in the connection between your dust collector and your tool. Static pressure directly impacts the dust collector's ability to overcome resistance as the air travels through a pipe or hose. Longer runs, bends, corrugated or ribbed (as opposed to smooth) internal walls of your hose or pipe, and smaller-diameter hose/pipe are all detrimental to airflow, and the more of these issues you have to deal with, the more static pressure you need to produce in order to overcome them.
WOOD Magazine has a nice article that summarizes the common airflow requirements to capture dust from various types of machines.
Of specific interest to you:
- Jointer, up to 6": 350 CFM
- Bandsaw: 350 CFM
As I mentioned earlier, the diameter of your dust collection hose or pipe produces a loss of static pressure, and the aforementioned WOOD article has a table summarizing air volume through hoses of various diameter at a speed of 4000 FPM (feet per minute), the speed at which the airflow will keep the dust particles and chips suspended in the air.
If your most demanding tool requires 350 CFM and a 3/4 HP dust collector produces 650 CFM, you use a short run (less than 10 feet) of 4" or 5" hose or pipe. Keep in mind that any bends in the line, and even using flex hose vs. straight smooth rigid ducting, will reduce the effective CFM--so you should use as short and straight a hose as possible.
For a more in-depth overview of calculating dust collection requirements, see What advantages does a dust collector have over a shop vac?
I always wear a dust mask and going to upgrade to a respirator, would that be enough?
A dust collector's primary purpose is to prevent your tool(s) from clogging up with dust and chips. Removing dust from the air is secondary, and no dust collector can capture 100% of dust particles. Regardless of your dust collector, you ideally should wear a well-fitted P100 or equivalent respirator.