Pre-loading textures only loads "defaults" like terrain, objects (hangars, towers, trees, etc).
I don't believe you can pre-cache skins anymore, unless you have more than 2GB ram or something, the option doesn't show up.
You can remove the .res files for the skins you don't use, or don't use often. Then disable the in-game download.
Also, going from 1024 to 512, or from 512 to 256, allows you to store 4x as many textures as before in the same space. If you're running at 1024 (and quite a few are, just because they can) then drop it down to 512 and you won't notice much difference. Going from 512 to 256 you WILL notice a difference, but you have to balance "looks" and "performance" in that case.
EDIT: P.S. A few folks had stutters and did a full disk defrag, and that fixed it (sped up their read time, to the point it doesn't stutter)