Hi Alex,
Most manufacturers claims for lumens can be reduced by around 40% for true real world lumens - I measured my 900 lumen Optoma H77 at 510lumens for instance.
You also need to set up the projector to the source - in the case of a DVD player that's relatively easy to do - you use a test disk like Avia for NTSC or Digitital Video Essentials for PAL to set the black level (brightness) and white level (contrast). That way you should be seeing all the detail available without crushing it into large areas of black or white for example. Failing that, use a THX DVD that has a THX optimode on it - it;s not as agood as a pukka test disk, but it's better than nothing.
PC levels are different to video levels - PC levels use a digital range from 0 to 255 whilst video levels use 16 to 235, so the PC can seem brighter for two reasons - one is the white level is brighter,. and two, you may have your white level set incorrectly (too low). A PC can give a far better DVD image than any stand-alone player though, so stick with it.
A screen can help get the best from your projector, but the screen can be compromised by any ambient light in the room. If you have ambient light, a grey screen can help, as can a screen with gain. A grey screen with gain like the Stewart Firehawk or the Da-Light High Contrast Cinema will be the best choices IMHO unless you can remove any ambient light from the room.
Gary.
Gary.