You can find the 2 files . Modified branding.zip and data.zip. Both contain 480x800 and 800x480 folders. Check out the contents for yourself:
rename the files after Download to "data.zip" and "branding.zip" like the ones you already have with your igo8 installation.
Now go ahead and replace the two files in your igo8 folder with the new ones you just downloaded.
So much for the needed files ...
Now here is a basic sys.txt that will put your iGO8 install into highres WVGA mode, not many other settings/tweaks in there ... you can play around with it yourself:
Type="here put your device name"
That's it. With these settings iGO8 should start in native WVGA resolution. If you want to switch between Landscape and portrait, you can do so from within iGO8 settings.
Make sure you have this sys.txt file in your \ProgramFiles\iGO8 folder on your device if you installed iGO8 to your device storage. If ya running directly from SDCard, then you should only need the sys.txt file in your iGO8 folder on your sdcard.
I don't want to give a Complete iGO8 installation manual here. There are plenty of posts around that explain the procedure.
Go get your igo version, get the latest Teleatlas maps for iGO 8.3.5.x, replace data.zip,branding.zip and sys.txt with the files supplied in this post, then imstall iGO8 via autorun (2577 folder). That's the easiest way.
Edit: if you want to switch back to 240x400 / 400x240 then change highres=0 to highres=1. Doesn't make too much sense in my eyes, but highres=1 in this case means lower resolution .