If you use RGBA for modern browsers you don't need let older IEs use only the non-transparent version of the given color with RGB.
If you don't stick to CSS-only solutions, give CSS3PIE a try. With this syntax you can see exactly the same result in older IEs that you see in modern browsers:
div {
-pie-background: rgba(223,231,233,0.8);
behavior: url(../PIE.htc);
}