Hi, I have a pci-e 9800GT with 512Mb of ram and it only does 24bit as well.
24bit is ....bpp, bit per pixel.
commonly confused with 32bit processing.
from the wiki....http://en.wikipedia.org/wiki/Color_depth
15/16-bit color (High Color)
24-bit color (True Color)
30/36/48-bit color (Deep Color)
also from that page....
"32-bit color" is generally a misnomer in regard to display color depth. While actual 32-bit color at ten to eleven bits per channel produces over 4.2 billion distinct colors, the term “32-bit color” is most often a misuse referring to 24-bit color images with an additional eight bits of non-color data (I.E.: alpha, Z or bump data), or sometimes even to plain 24-bit data.
Systems using more than 24 bits in a 32-bit pixel for actual color data exist, but most of them opt for a 30-bit implementation with two bits of padding so that they can have an even 10 bits of color for each channel, similar to many HiColor systems.
your card is working at its optimum at 24bpp.
Cheers, Regards Glenn