I understand your "500 mania". I use mine for everything - internet, e-mail, playing music, games, printing, even emulating MacOS 7.5. It's the only computer I use at home.
I'd say the two greatest things you can buy for your A500 are:
Viper 520CD - for 2.5" IDE interface, 8Mb 32bit RAM, 33Mhz 020 and 3.0 ROMs
DCTV - 24bit composite image display, video capture and paint program