it's a epson powerlite s5 HD-Ready — supports 720p, 1080i

I'm wonder how much crisper the resolution will be. will it be even noticeable?

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Answer:
I find that the colors are superior and the image is crisper when viewing a true 1080p (or 720p) source rather than an upconverted image or lower resolution image such as DVD.

It also depends on the DVD or Bluray. Some look better than others.

You can really see it on a huge screen (42″ ). You might not see much difference on a small screen.

I would rather not watch a DVD on my 90″ projection screen. I use a PS3, which is a fairly good upconverter, and I see a massive difference with Bluray over upconverted DVD. My projector only goes up to 1080i also.

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Answer:
I guess my question is, what are you feeding it now? If a good DVD looks bad on the projector, either your DVD player leaves something to be desired or the Epson isn't a very good projector. As it is, the Epson won't support 1080p, which is the ideal resolution that a Blu Ray player can put out.