Well, it depends on what you mean by mod. Sure, there are some common expansions...
1) Of course, you practically HAVE to have the baseboard 512k expansion to bring the system up to 1MB.
2) Next, adding the Fat Agnus and a NTSC/PAL switch is a nice touch.
3) Maybe throw in a sidecar hard drive and some additional Z2 Fast RAM.
4) If you have Z2 Fast RAM, you might want to consider a jumper to switch your 512k baseboard expansion from Ranger mem to Chip mem. (Allowing you to have 1MB CHIP RAM instead of 512k Chip / 512k Ranger)
Really, I'd only consider 2 and 4 as mods, though. The other two are expansion devices. ;-)
Also, (and no offense to the people who love them) but I never understood why you'd want to put a processor card in an A500. It drops system compatibility down around the level of an A1200 or worse. And the A1200 with an '030 card ends up being both faster and cheaper. Part of the charm of an A500 is the blistering 7.14mhz. :lol:
