I'm having trouble with trying to export to bmp:

Whenever I change the dpi, or height/width in pixels when I'm exporting, the other two values (out of the three) change automatically. How can I set that I want EXACTLY 72.00 dpi, 480 x 320 pixels? (Changing the page size in "Document Properties" doesn't seem to help...)

Inkscape's (SVG) default is 90dpi, so you need to do a little math to see how big the drawing needs to be in Inkscape to get your numbers when exporting:

x=480*90/72=600
y=320*90/72=400

So make your page/drawing 600x400 px and when exporting change the dpi to 72

Thanks--all I need to do is multiply 72 dpi x 1.25 - 90. I just wondered if there was any better/easier way...but I guess this is pretty easy.

Yup you got it. No easier way, that's what everyone was talking about in school when they said you needed your maths