First of all, almost all posters use standard 100~150dpi, max, sometimes they will request as 200dpi only, so a 8mp enlarge to 11x16 by using 250 dpi is far enough for a poster.

Only case that require large size with 300dpi is the "slide film" prints, which I don't think it's the case here.

as to 8mp image of 20d, actually it's basically a 6mp 10d chip modified. the image carries a little more detail, but nothing really more than original 6mp ccd can provide you.  My suggestion would be that if you really want to enlarge it, you might have to try some enlarge programs like FM filter LSpro or something like that.  They will tend to sharpen the "contour line" more, to make image stands out at distance viewing.

Shooting RAW will give you much sharper image, and so is TIFF (use TIFF instead of JPEG) for prints.  However, 10d and 20d did fairly well on JPEG, actually you won't see too much difference if you print in small size.  But the answer to your case, you should shoot RAW.

213 dpi will enough!Even 180dpi can get you bigger picture 'cause ypur eyes can't tell
RAW or teff can get better quality