Hi !

Try using textures for ground and walls not bigger than 512x512 and please use only textures, wich size contains the following values :
2,4,8,16,32,64,128,256,512,1024 !
Downgrade your 800x600 texture to 512x384 ! I`m shure this will work and will still look acceptable.

Almost all my textures are 128x128, some are 64x64, 32x32 and in some cases I use 256x256 textures. I think the results are quite nice.
Please visit THIS WEBSITE and check out how my project looks like. The site is in german. You`ll find screenshots in the category Planet UD46.

I hope you understood it all and I could help you.

Best regards!