How can I download from iTunes at 320KBPS?

Sounds like you followed the procedure but here's what it is anyway.
1 Open iTunes
2 Go to iTunes>Preferences>Advanced>Importing
3 Choose the format you want from the menu alongside Import Using, for instance AAC Encoder
4 Choose Custom from the Setting menu
5 In the dialog box that now appears, choose Stereo Bit Rate 320 from the list
6 Leave the remaining two boxes at auto and click OK
7 Click OK in the Advanced box to complete the setup

Worked for me, my import setting is AAC at 192 kbps

It sounds like you are talking about files that you download from the iTunes Music Store. Those are only available as 128 kbps protected AAC files. The "import" preferences in iTunes are the settings that iTunes uses when ripping songs from a CD.