Video is 72 dpi no matter what, including HD but HD has more lines to work with than SD. Also the difference between an interlaced format and progressive formats are significant here. You'll get all kinds of difficulties with resolving small type in interlaced formats. So setting the timeline for a progressive format at a high HD resolution is the first step. Or what about just using PowerPoint to do your slide presentation?
Greg Ondera
http://www.Plexus.tv
http://www.SurgeonToday.org