IT service start-up gateway & merchant service provider selection?

I have volunteered to stand up an accounting system for a fledgling technology services firm that just landed its first two income-producing contracts.  I've chosen Quickbooks Pro 2010 as the accounting software, but I'd appreciate tips and recommendations regarding how best to choose a web-based online customer payment service provider (with ease of QB integration as one of the selection factors).    Part of the new company's income stream will come from individuals who sign up for an optional "add-on" service that augments the core package provided and paid for by their parent organization.   These individuals will typically pay $25 a month by credit card, and they will be free to stop their subscriptions any time they want.  There could be anywhere from 5,000 to 50,000 such individual customers a month.   

If you were small and starting from scratch on a restricted budget for a client likely to grow rapidly in terms of credit-card payment transaction volumes, with only Quickbooks as a given, what gateway and/or merchant services provider would you use?    Thanks to all who are willing to share experience and tips.