HD projector with (720P, 1080P) still expensive here.  u may try those normal  HT projector from brand like infocus, BenQ, Optoma and  cost below 4K, there are good enough for normal DVD movies. 

just FYI, they worked perfect if the screen projection less than 85" diagonal length (X) and viewing distance ard 1.9X-2.3X.

<Once everything in HD, are the projector still can be use in HD mode?>
Sure OK.

I think manufactureres will try to sell you anything, some even try to market multimedia projector as HT projector.  So to qualify as a HT projector, the following are some things to look out for:

1. At least 480 lines of resolution, why... standard DVD has 480lines of visible pictures although it actually carries 525 lines.
2. Contast ratio of at least 5,000:1, why... better colour and pop to the picture, the higher the contrast the better the picture (this is a general rule).  Though most of the budget range of HT Projectors, my TW700 cheat by using a single or double iris to reduce the light output to get good contrast ratio numbers
3. Lumen count from 800-2000, why... for viewing flexibility, can watch day and night
4. Zoom lense of at least 1.5x, why... for placement flexibility
5. Low decibel fan 25db seems to be about the idustry standard for acceptable HT projectors.
6. Compatible with HD720 and HD1080 so that u can use it with the new formats
7.  Good warranty and bulb replacement (usually first 3 month)

These figures will move up and down based on personal preferences.

Jason