You should convert your textures to dds, use the free Microsoft dds conversion tool or the other which name I forgot. DaBrain knows the name.
(My personal favorite is still Photoshop and NVIDIA DDS plugin)
Memory usage:
TGA, Jpg* 100%
PCX** 50%
DXT5 25%
DXT1, S3TC 12,5%
*Both have the same size when loaded.
**Is used as 16-bit format when loaded
Also, DDS does not require any extra processing from CPU, unlike tga, jpg and pcx
Now you know the secret why later versions of the Base runs much smoother than the original 3.0, even though so far effects have improved more or less in each release after 3.0.