A standard DVD, unless up-scaled will look alright on a HD-TV but won't have the same effect as it in its original scale.
An Up-Scaled version will look tonnes better, and its definietly still watchable.
Blu-Ray and HD-DVD simply up this some more.
People in Europe, using PAL will notice less of a difference than you lot in the NTSC territory, especially when running in 720p.
A lot more detail is noticable though, and of course, it doesn't look at all stretched.