How to convert Latitude/Longitude to XY coordinates?

Hello,

I need to convert pairs of Lat/Long positions into XY coordinates so I can represent them on screen (size 1280 x 800).

So for example: (47.525162,-122.265599) to (652,249) --- (or whatever it may be?!)

Any ideas? This does not need to be a fully accurate representation of course, but the closer the better!

Thanks,

Joe