640Kb should be enough for everybody... seriously tho, 3Gb is a lot of RAM. If a bitmap is fully unpacked then its about 4 bytes per pix x width x height x layers + undo buffers. Which is a lot but even 5Kx5Kx4bytes = 100Mb so you would need 10 layers of that to get to 1Gb. Lack of RAM is the usual culprit why stuff slows down. For Gimp and most graphics apps then video card ram makes no difference at all. 3D apps, my VDale, and the odd bits of Photoshop + games make strong use of graphics card ram but its more uncommon.

When I tested image file sizes I usually get to about 12Kx12K before it starts to die in the file formats. By about 20Kx20K they mostly start crashing and some cant take more than 16K. The PS own brand format and TIFF are pretty robust at size but if your not mental like me then using something about the 4K size should be no problem for any map in Gimp with a fair amount of ram.