Lately I have been goofing around a lot with terrain mesh. The mesh for the US found at http://www.simviation.com/fsdterrainusa.htm
is pretty good and I have not had problems with it. While it's not the best, it's better than the FS default mesh and it's just about seamless. I recommend it for freeware and if you download it, you don't have to worry about it's borders if you stay within the mesh. There is better mesh out there for specific areas that is freeware, and even better payware mesh and it all matches rather nicely.
There is one small con that I have noticed with higher resolution mesh. At some airports, it appears that the flattened part of the airport sits on a "plateau" a couple of feet high. Sorry, I can't post a screenshot because I am at work right now. Anyway, I think it's tolerable.
Your system does not have to be super powerful to handle more mesh. Give it a try and let us know.