What would be the ideal text for a donate button? My client would like to use "Give" as the button text, but should I urge for "Donate", as I feel this is more of the convention.
'Donate' is a hyponym of 'Give', in this context it is the choice that does exactly what it says on the tin.
Also, the term 'Donate', to me, suggests a formal agreement between two parties. Using the term 'Give' may undermine that formality, and imply a less than reputable nature, which may in turn affect someone's decision to donate.
I agree with kaiser's recommendation of using an A/B testing tool. You're guessing without some data to back it up.
There is an article in the May 2012 issue of Wired magazine on A/B testing. It referenced some examples from a fund-raising website for supporting Haiti's relief efforts. They started with "Give" or "Donate", but eventually found that "Support Haiti" was the best performing text for their cause.
I think some user research might be more helpful than choosing between "Donate" and "Give". You could A/B test to find which one is the best of those two but that doesn't mean there aren't some better options. Survey or interview some potential users to see what they would expect to find and research some similar websites to see how they implement donation buttons.