are you going to implement some sort of level of detail algorithm? I assume you do not use any yet, but when you increase the size of the landscape if you do not manage the number of vertices you use the performance will drop straight down to crawling. if you are planning to use some kind of LOD, it is, in my experience, much easier to start with that instead of lightning, texturing and blending implementation, because these stages depend a lot on how you implement your level of detail.