The Vivitek D556 has a maximum resolution of SVGA (800x600). That's 39% less resolution than a XGA (1024x768) projector, which is the most frequently found resolution.
A SVGA (800x600) native projector resolution is in fact the worst resolution amongst all projectors of a comparable age and type to the D556.
Amongst similar types of projector the best resolution is 1080p (1920x1080).
It may also be worth also looking at one of the other projectors that have a resolution of SVGA (800x600) currently available including the Infocus IN112A which has a better Rating; or the Infocus IN122A (£300.42), or the Vivitek D552 which is cheaper.
If the resolution of this projector is not high enough for you then you might also want to consider the Infocus IN124 (£286.45) (XGA (1024x768)) which is also higher rated.