We currently offer a massive range of downloads for FSX, as well as older flight simulator add-ons for the ever-popular FS2004, FS2002, CFS3 and now becoming ever popular with dediced virtual aviators is Laminar Research's X-Plane series.
The best trade off for performance with authenticity is the 1024 x 1024 size textures. 512 x 512 can be done but you lose a lot of detail as the file size is smaller, hence less detailing possible. I like working with 2048 x 2048 size files personally, but you need at minimum of a gig of ram and a good 128 mb vid card to render the textures in game with any decent quality. Preferably a 256 mb vid card of 8x AGP or PCI-e rating to get good fps and avoid the "slide show" effect as the system bogs down with art work from the extras like effects, and scenery, and the other planes that have to be rendered.