MadeinAlaska, I am doing exactly what you're talking about. I threw away my Nikkon scanner (the Scuzzi card with it blew my motherboard.....!!!) and bought a Microtek Scanmaker i900. It'll do negatives up to 8x10 and does a superb job of restoring old b&w 620's from 60 and 70 years ago. It will scan up to 12 slides at a time. It also copies reflective material as well. However, as Roger pointed out, converting slides is VERY time consuming. My slides go back as far as the 50s and 60s and while they have been kept in very good conditions, they are certainly not all exposed perfectly and I'd say about 85% of them need some diddling. Enter PhotoShop Elements. Add contrast, fix the color, sharpen ones that need it, crop out grandma's leg - if you're any kind of perfectionist with your pictures, you'll be hooked on making them look even better than when you took 'em. Trust me. I scan them at at least 300% of the original size and at least 400 dpi resolution. If it looks like I might crop it down some, I scan at higher resolution and size. You could crop using the scanner software, but slides are so small it just seems easier to do it after you've blown it up a bit.
Here I depart from some of the others. I don't take stills anymore - I've been doing HDTV since the first consumer camera came out several years ago. I use Sony Vegas Pro 8 and Sony DVD Architect programs to convert the tv to DVDs - ah, soon, HD DVDs, Pro 8 has bluray capabilities - and so I use these two programs to put the slides on a 50" old Pioneer plasma. These two programs are bundled and sold as a package but it is quite expensive and you could do the same thing with Roxio's package and perhaps others less expensive.
What I like about using a movie maker package is that you can add titles, you have the full panoply of transitions (and you can use the ones YOU choose, not some random slide show), you can zoom in on anything of particular interest, you can add music easily and if you wish, even narration. And trust me, making the final "movie" takes far, far less time than scanning all those slides.
I think the downside no matter what you do is that your "portrait" slides are going to be small compared to everything else. And I guess there's just no way around that. When making the movie, I try to lump a few portraits together so I'm not continually jumping from horizontal to vertical and back and forth.
Another thing I've done a bit of is cropping. You'd be surprised how many of your landscape slides can be cropped to 16x9, giving your slide show a more "high def" look. And if you're already diddling each one in Photoshop, it only takes a second to set up a custom 16x9 crop and then only a couple of seconds to crop each one.
Just ideas from an old guy.....