Converting between coordinates

This should possibly be in the developers forum, so I apologise if I’ve posted in the wrong place.

I have a large dataset of coordinates in EPSG:4326 and I want to convert them to EPSG:900913 (or 3857?). I’d like to do this conversion in Excel/OpenOffice because I need to do various different things with them. Can anybody point me in the direction of a formula that will do this? Also a reversal formula would be great.

The other option I have is to put them all into a PostGIS database in EPSG:4326, import that into a .shp file, convert that into EPSG:900913 and then export that back into PostGIS, then create a .csv from PostGIS and import that into OO. Not ideal!

Thanks for the help.

Have you already tried http://wiki.openstreetmap.org/wiki/QGIS ??

It can deal with different projections and has interfaces do databases, if I am not wromg.