and 1GB only fits just over half on.
PS
if you aren't going to use it off road, I reckon that City Navigator is more useful. The contours make to roads quite hard to distinguish when riding, and they also change the display scales at which minor roads are displayed. With CN, minor roads are visible up to a scalebar of 1.2km (screen 5km x 8km (ish)), but with TopoGB they aren't visible over 300m scalebar. This is based on use with a 60CSx.