I believe the one I see pretty much every weekend (actually, it's more like every other weekend) is a Faroudja professional-level upscaler that looks pretty damn good, but is several years old technology at this point. It's probably comparable to the Faroudja upscaler that Masterlu mentions in his first post.
You may also want to look at the Lumagen HDP, as it's several hundred dollars cheaper (US$1199), and looks like it has been the recipient of trickle-down technology (though don't hold me to it, as I have not yet confirmed this) -- notice how it has "SD and HD per-pixel motion adaptive deinterlacing" listed as "newly added"?
Dew et -- it will revitalize you DVD collection, and render upgrading to HD moot.