There are mail order services that do this for a very reasonable price
and at a seemingly very good resolution.  You ftp (or other trasnfer)
your image to their site.  They produce the 35mm slide and mail it to you.

One such co.: http://www.slides.com/

I've been tempted, but haven't done this to date.

Digital projectors are 1024x768 max at the under $10,000.00 range.
That's 768 pixels high for portrait images!

Many art shows still require slides rather than digital files for entries,
and there are labs, both on the web and brick and mortar, that deliver high
quality slides.