Audio production technique's explained for lead vocals.
In this clip we see how the music producer literally "tunes" the reverb on the lead vocal for a very interesting production trick.

Watch as Pro Tools guru Kenny Gioia guides you through creating a Vocal Exciter from scratch, within Digidesign's Pro Tools. This video is taken directly from Kenny's "Pro Tools Tips and Tricks Volume 2" series of videos, available at Grooveboxmusic.com

Danny Byrd (Hospital Records) shares some of the vocal tricks he used to create "Shock Out" and other tracks on his album "Supersized."

Music: Shock Out & Weird Science by Danny Byrd

Here is a quick video about recording vocals in multiple takes.

The microphone on my wish list right now is the Rode NT2-A. It has won enough awards to get my attention. There is lots of Audio Tehnica that could do the job, but I do suggest finding a microphone with selectable parameters. This will make the microphone more versatile in the studio so you could use it for other things with the different polar patterns etc. Let me know what you end up doing.
Adam Smith

i just bought an Effect processor(Lexicon MX300)to be used in my church.

I want to use it to enhance the worship leader's singing experience. I wish to be introduced with some programs and recommended setting for a start for mixing some effect into the vocal.

Later on, i will also be using it on an acoustic guitar.

Thanking you inadvance
Johnny

Did the Lexicon come with an instruction manual? I have used that unit before. It is a nice piece of equipment but I did have a hard time navigating through it. the Lexicon MX300 is mainly a reverb unit. It can make your vocalist sound like it is singing in many different places. ( a Hall, Stadium, Bathroom, Club, Etc) Reverb is one of the best tools for making the vocals sound "larger then life". There should be many presets that come with the unit. You should plug in a microphone and do some tests and find some patches you like. Then get someone to sing through it and find the best patch for the room your in. I don't know the room that you will be singing in but make sure the effect is not to over bearing. You can mainly control that with 2 features that are in this unit.

1. Wet/Dry Mix - The could also be called the effect level. The reverb of the room your in plus the reverb in this unit could drown out the singer, make sure there is an even balance. 
2. Pre Delay - Lets say that you were testing the unit and the pre delay was set to 1 second. If you said the word "test" in the microphone. You would automatically hear yourself say the word through the speakers, but then 1 second later you would hear the word "test" again with a reverb effect. Play around with the pre delay until it sounds good in the room.

There is one way to test if your settings are right. Get someone to sing through the microphone while you stand at the back of the room. If the vocals still sound good there your doing something right. Remember that a empty room sounds different then a room full of people.  If you did your tests in a empty room, you might want to turn up the final effect to even things out.

Best of luck, let me know how things turn out

Thanks for the question.

It does sound like you have the right idea for microphone placement. My only concern would be the pre amp. If it is noisy you would want the microphones as close as possible to the choir so you won't have to turn the pre's up too loud. I would place the microphones above the choir (around 2-4 feet) pointing down towards them. As for placement a good start would be the 3-1 rule. For every foot the microphones are away from the choir, place the microphones 3 times that distance apart. For example, if the microphones are 10 feet away from the choir place the microphones 30 feet apart. This will help with phase problems, even if someone is listening to your recording in mono. I would place the microphones close to both ends of the choir and point them towards the center. With the size and shape of your church I don't believe you could make a perfect XY and keep the 3-1 so try to get as close as you can. The main focus of recording a choir should be .... well... the choir. Do some test recordings and then get the choir to stop singing. Listen back and make sure you have a even recording of the whole group. You might need to try this a few times to get it right. Best of luck, let me know how it ends up. 
Adam Smith