Render few images and stitch them together in Photoshop?
Try by selection region and split them in 4-5 portions they must overlap somewhat, save every region separately, then when you are done with rendering you will need PS or some other image processing tool to stitch them.
Ver1 PS: Open all images, add them as layers over one image(no need to resize canvas etc. it will do it automatically when aligning) select all layers and go to edit tab/auto-align layers, leave it on auto but you can choose from options what type of align you wish to be applied and after that you will end up with rearranged layers with layer masks, again select all layers(they will be selected already but in case you deselect them...) choose Edit/auto-blend layers to equalize everything, you can then flatten down all and if you done everything properly you will end up with one image that is stitched without visible seams. This requires regions to overlap... I think that PS had limitations to 40%(Didn't use PS for this long time)
Ver2: Stitch them by hand with layer masks but you will need to blend them "manually" to equalize layers.
Ver3: Use PTGui stitching software(best solution for stitching images)
Ver4: Buy more ram
Ver5: Convert scene objects to proxy to save memory.
Ver6: There is plug-un for PS called Alien Skin Blow-up for up scaling images, and it's pretty good but don't get you hopes high.
