Firstly,  what are ... DATA PROJECTORS ?

Data Projectors or Multimedia Projectors ...basically mean the same thing.
They give you big screen images of whatever is displayed on your Laptop computer (or PC) screen.
These projectors connect via a cable to your computer.
Using popular presentation packages such as "PowerPoint" or similiar,  you can display a presentation via a data projector and ... you can save hours associated with the preparation and printing of older style overhead transparencies.
Plus ... and this is the most important point ... your presentation will be much more visual and dynamic.
So why's that important .... because ... your audience only retains 10% of what they hear.
But (and it's a BIG but) they retain over 50% of what they See and Hear !

The need for Data projectors in our modern worklife really is obvious.
Don't just tell people about your plans ... get them visually involved and excited !

Some basic facts ...

When choosing a data projector you must try to remember that the output from your computer needs to be compatible with that of your projector.
This output is often measured in terms of screen resolution (or 'pixels') ... that make up the image.
Some common references for screen resolution are :

VGA      This is 640 x 480 (pixels - or dots per inch)
SVGA    This is 800 x 600 pixels  - 
XGA      This is 1024 x 768 pixels - 
SXGA    This is 1280 x 1024 pixels
UXGA    This is 1600 x 1200 pixels

How do I find out what my screen resolution is ?

Go to the Control Panel on your computer and double click Display/Settings.
Or 'right click' on your mouse button, go to 'Properties', then select 'Settings'.

Everyone's talking about Lumens ...what are Lumens ?

All Data Projectors specify their brightness in terms of ANSI lumens.
(ANSI refers to an American association which ensures that all comparisons are valid).  The figures often quoted by many manufacturers are however "best case" scenarios.
The brightness specification is also based on a new bulb. Generally though ... the higher the lumen output ... the brighter and larger the image is.
In the year 2000 ... most projectors were rated around 500-800 lumens. Today's (2008) entry level lumens start at around 1200 lumens.

So ... how do I choose the 'best' data projector ?

Here's a quick guide which hopefully helps you make a more informed decision

Determine what size of group you are normally presenting to.  Is it 6 or 600 ? If it is say less than a dozen people most of the time ... you will not need the most expensive model. Generally something around 1500 lumens would be suitable.
Determine where you will be presenting most of the time.  Is it in a boardroom will full sunlight and powerful internal lights ?  Or a small meeting room with no windows ?  If you are presenting to only a small audience ... but in very bright locations ... a minimum of 1500 Lumens should be considered.
Are you going to travel with your data Projector ?   Generally look for Data Projectors that weigh somewhere around 2-3 kgs. Projectors around this size easily fit hand luggage requirements on planes.
Are you Training or Selling ?   Generally if you are training people via something like a PowerPoint presentation ... SVGA resolution is probably adequate.  If you are displaying lots of spreadsheets or architectural drawings with fine lines you probably need to consider XGA resolution as it offers more pixels .. for better clarity.

And finally, how far away from the projector is the projection screen going to be ?  Remember, the further away the screen is, the dimmer the image is, regardless of lumen output.
1200 ANSI Lumens .... fair brightness for small groups in small rooms. Can be a bit dim in brighter rooms
1500 ANSI Lumens .... bright, clear images for larger rooms with lights on
2000 ANSI Lumens .... very bright and ideal if meeting rooms have direct sunlight coming in
2500 ANSI Lumens .... ideal for large meetings rooms with many people

There are many other things to consider before making your purchase.  And without sounding evasive ...
a lot depends on what else you want to do in your presentations.
Some things to consider include :

Are you using sound in your presentations ?  The sound would generally be played back through your computer .. so consider how many people are in the meeting.
Is the data projector both PC and MAC compatible ? (nearly all are ... but check anyway)
If you wish to run a DVD through your projector .. can it distinguish between different formats ?  (PAL, NTSC, SECAM video standards ... important if you travel overseas)
Does the projector offer a full function remote control ? ... so that you can control the presentation from anywhere in the room)
Does the projector have a manual or a powered 'Zoom Lens' ? A manual zoom means that you will have to always be near the projector ... rather than using your remote.
Does your Remote Control handset (nearly all brands have a remote ... but functions do vary) have an inbuilt laser pointer ?  It's a good idea for presentations ... and if it's built in ... it's just one less thing you have to carry.
Does the projector come with good adjustable legs that let you tilt the projector, without having to stuff phone books under it ?

And finally .. how about the convenience of a wireless projector ? Wirelessly connect your projector to your PC or laptop. No more tangles in the meeting room ! See the new Panasonic wireless projector (PT-LB51NT)