I have been using 8.562 for a while now and it works amazingly well (about 5-7 MB of RAM left). You can find the download link here.
If you want you can still use the latest navcore supplied by tomtom for the go 500 (which i think is 7.9x), and install all of the latest maps (885 for the moment) which fit the criteria by patching your navcore/map using a tool like FastActivate. Of course the UI is ugly.
It's up to you really.