In the early 80's when my cousin got married, I used my Harmon Kardon HK 330 (30 wpc!), a Harmon Kardon tapedeck and some Dynaco A25 speakers. It was a large venue 300+ seating but the dance floor was very small and they didn't expect people to stick around. it worked out pretty well if I recall.
For my wedding a couple of years ago. I rented four pro speakers on stands, mixer, wireless mics (300 - 400 seat venue). I used my own Crown power amps (550 wpc). This was to provide, sound during the ceremony (I and the clergy were mic'ed) background music between the ceremony and reception and during the reception (no dance). Source was my notebook and an portable MP3 player as backup. It was very inexpensive to rent. The only hassle is transporting the stuff. Here are some tips:
1. hook everything up before hand at your house and draw diagrams on how to hook everything up. Have your groomsman hook everything up to practice expecially if they are not familiar with how it is done.
2. bring lots of power bars and heavy duty long extension cords as you never know where the outlets are going to be
3. don't use powered speakers for the reason above. Use passive speakers (ie you need a power amp) as this will allow you simplier wiring and speaker placement
4. bring gaffers tape to hold down the wiring so it doesn't become a tripping hazard.
5. have a backup for your source and ideally your mixer
6. ideally, the simpler the setup the better ie don't rent a 36 channel mixer.