The antenna type you're probably looking for are called yagi antennas; directional antennas to help focus your signal. They can be a pain to aim sometimes though.
Make sure you have proper grounding or you're going to have a nice and expensive lightning rod.
First I add that some of these replies need clarification.
There are two kinds of overlap, one is channel overlap where the frequencies overlap, and the second being signal overlap.
You MUST have signal overlap for all devices to have coverage in all areas, or even most devices in most areas.
Secondly, there are various schools of thought for frequency overlap and some manufacturers even suggest putting all APs on a common channel. In the case of roaming IP phones this case becomes even stronger as a phone may hop across APs while in a call. This of course depends much on the hardwae of the phones and antenna placement and design.
Let us assume that we had a large open area that we wanted wifi coverage in. Now lets take a pole and place it in the middle of the area. Now we place 4 directional 90 degree antennas on the pole, each 90 degrees from the other . In this situation one may make a strong case for having all APs on the same channel to facilitate roaming. In theory there is little signal overlap but all frequencies overlap.
Now we have an open area with walls on four sides. and place an AP on each of the four walls. The signals WILL overlap from each of the 90 degree antennas , so we may want to consider using separate non overlapping channels on each AP , however there are only 3 non overlapping channels. 1, 6 , and 11. So instead we do the best we can in North America this might be 1, 4, 7, and 11 , each AP having SOME necessary frequency overlap. Of course in a perfect world this might be better accomplished with three APS in a triangular configuration.
In my home I have toyed with APs on Same channel and separate channels and in the end I see little coverage difference., I do see however that some devices such as wireless IP phones can more easily hop to another AP while in a phone call. I see that in most areas I do not have more than 2 overlapping signals and each on channel 4 at present. As I sit here I can launch wifi seeker on my android and see either of the 2 available APs and even connect to either. This of course is easier to test with separate SSIDs but more practical to use common SSIDs fopr everyday use.
Best Answer
Your question is actually very general. The Right AnswerTM involves the number and location in the building of expected users (lots in a sanctuary or classroom, few in the office area, etc), the logical separation between different types of users (public Internet users versus office users who might be accessing LAN resources), the physical layout of the structure (for AP placement and cabling concerns), and knowing how to perform wireless site surveys with whatever APs you select (depending on your budget and necessary features) and test client devices (because the antenna characteristics of a laptop are very different than an iPod Touch, etc).
It sounds like you're coming at this from a level of understanding that will cause you to have a very, very steep learning-curve. You may want to get a professional with experience in these kinds of deployments to come in and give you some pointers. If you're looking at supporting more than 20 - 30 active clients in a large open room then you probably need professional assistance. There's a pretty vibrant "church IT community" out on the 'net, and you may be able to enlist some support there.
I'd be wary of using wireless "repeaters" if you want any significant number of users active at any given time. Wired back-haul from multiple access points will give you the best aggregate bandwidth in the air. That means running cables, so you'll want to be certain you've tested your planned placement for APs to be sure they give you the coverage you need before you start running cables.