I have a Epson 1640 that is a flatbed with Transparency Adaptor that will scan up to 4x5. It does a pretty good job (even with 35mm for web display) and there's newer/better versions. I think the model is called a 2450 or something... you could search photo.net, lots of discussion on the topic there

I must add that I don't print anything B&W via scanning, but I have printed a few colour things (on a Canon S900 - A4 sized) and they have come out pretty good from 645 negs.