TGA's take up a lot of memory, quite a bit, actually.
Ok, say you have a 1024x1024 texture, this means you have 1,0242
pixels or 1,048,576 pixels.
Ok, TGA's have 8-bit color, which means they have a total of 256 bits each for Red, Green, Blue, and Alpha. So multiply 256 * 4 and you get
1024 bits per pixel
See where this is going? Muliply 1,048,576 by 1,024, and you get 1,073,741,824 bits (1,024 MB) used for your TGA... not including the TGA identifiers.
I think most of the modern GPU's won't care at all, but the older ones will have a heart attack if you use too many of them... I also think that the OpenGL 2.1 GPU's had a maximum of 256 or 512MB for RAM.
P.S.
TGA format supports Run-Length compression, which helps mainly for images that have areas that have the same color value.
Also, there are other DDS formats out there.