yes a top mounted fan will be more noticably noisy,but depends on the fan itself and how quiet the actual fan is. i think most cases can be made to have positive pressure.if you have more cfm (cubic feet per minute) going in than you have coming out that is positive pressure.put simply,more fans as intake than exhuast create positive pressure.i have two large vents on the side of my case with no fans and if i put my hand beside them i can feel air coming out.thats positive pressure. you dont necessarily need lots of fans to do this either.i have one larger fan on the back as intake,and my psu fan as exhaust.not to mention the cpu and gpu fans. it will depend on your system how many fans you need and where they are placed.
The fact that you have those openings in your case means you do not (and can not) have any sort of pressurization. You will need to cover those openings (duct tape on the cheap) if you want to achieve positive pressure in your case.