could also go with a lighter coloured suit and a short sleeve dress shirt/tie. suit pants are usually pretty thin anyways (at least mine are) and a lighter colour won't attract heat, if you end up outside you can take the jacket off and be cooler in the short sleeve dress shirt with tie. then bring a long sleeve version of the same coloured shirt to switch into later when it cools down or you're inside.
oh and my last bit of advice (I got this idea from my grad) get a little sampler of the cologne you'll be wearing, and keep it in your jacket. that way if you do sweat quite a bit you can hit the bathroom, wash your face and give yourself another shot of cologne so you can keep macking on bridemaids without reeking of sweat
