but if i load all segments at once what difference would it make over having it as a piece or is it about blitz not rendering the segments off screen?
I'd say load them all, unless you have thousands and thousands, then you might want to think of another way, but generally, load them all in one go.The 1536x1152 map for Magicville uses a bit less than 7MB of graphics memory. I don't use more than 2-3MB on top of that (on the map screen) for other stuff, so I know I'm good to go on a 16MB graphics card.
You need to decide what your minimum hardware requirements are going to be, then base all your calculations on that. A 512x512 texture equates to about 1MB of graphics memory:
maxMemRequired = (imageWidth * imageHeight) / 262144 (512x512)