Wedding -what to buy, flash or lens ??

I've been asked to do the photos at a mates wedding, and just wanting advice on what to buy.

Current kit is:

350D, Kit lens, 50mm 1.8, 70-200F4 and a good tripod

Question is should I buy a flash like the 430EX or would i be better with a faster sharper lens to replace the kit... maybe a Tamron 17-50 2.8 ??

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