From the Buddhist perspective, is there an inherent conflict between being a believer in a theistic religion and a practitioner of Buddhist meditation?
According to Dalai Lama (targeting general public), if Buddhist ideals appeal to you, but you are already a member of another religion and worried about compatibility, you can (and should!) stay with your religion. From his perspective, all religions declare the same basic values of compassion etc. and if you practice your religion well, there is no point for you to switch.
According to Chogyam Trungpa however (targeting serious practitioners), there is a fundamental difference between theistic religions and Buddhism. Theistic religions posit a notion of Other Power (God) that you have to appeal to for your salvation. While the nontheistic teaching has practitioner depend on his own power (with Guru's help). From this perspective, the notion of God as an external entity must be transcended before the practitioner can fully reconnect with his/her fundamental sanity.
In Pure Land Buddhism practitioner initially depends on Other Power (Buddha Amitabha), which starts as an external entity, but eventually, through a change of perspective, is identified as one's true nature. So we could say, on advanced stages the notion of external God is transcended there as well.
Some theistic beliefs could contradict with Buddhist views. For example, if you believe that God is only savior, that is contradiction of belief in karma. Or, if you believe that God judges your experience, that contradicts with karma again. Or, if you believe that teaching given by God is best, that is contradiction with refuge in Buddha as best teacher of gods and men. When you deny, in such a way, right view and Buddhist refuge, you probably not practice Buddhist meditation correctly. But, if you just practice wishing of happiness to all beings (metta), that's not necessarily contradict with some God is Love idea.
Volkov's answer is on point, there is no "other power" in Buddhism and the Buddha indeed affirms that a person possesses agency; re http://www.themindingcentre.org/dharmafarer/wp-content/uploads/2009/12/7.6-Attakari-S-a6.38-piya.pdf