Originally posted by amoringello Unless you want full room photos of the venue, 20 is going to be pretty wide.
35mm would work better for shots of small groups. I would go longer if you want more intimate photos of the subject, cake, candles, etc... much like a wedding.
ISO 400 film is likely going to require flash if your indoors, unless its a daytime event with plenty of lighting.
I've only shot anything serious with digital, and I have to say that getting a fairly clean image at ISO 2000 is nice. Being able to go to ISO10,000 when necessary and still get useable images is amazing. Honestly, I'd be terrified to be limited to Iso 400. It is certainly what had been used for many years, so it's doable.
I applaud your abilities and willingness to use film. :-)
ISO 400 is passable with indoor lighting, if you have a fast lens (like 1.4). So, if you want to avoid too much flash, 800 speed film seems like the only real option in color.