It seems to me that what you want to gain from your A500 is not really sensible, unless you are upgrading an A1200.
The benefits of A1200 are exponential to the A500. If you opt for Custom Towered unit like myself, the possibilities are almost endless. My Amiga allows me to go on line and do most of the modern tasks of today, albeit a little slower of course.
The A1200 IMHO is the best overall Amiga system for upgrading and use. Although the A3K and A4K would be the dominate units in terms of rawness.
The A500 may have been the most popular but it is NOT the best for upgrading. Mediator set up for RTG and SC card set-up isn't an option on the A500. Also until SAGA is perfected AGA is also not an option and when it does come along, it wont be cheap. That stated nothing to do with Amiga hardware is cheap, yet it continues to remain popular. Just goes to show how good Amiga really is. I guess something's never change!
If you want a serious kick-ass Amiga 500 then your only option is Vampire! Although the purist are going to pan you, should you choose this option.
For an all round great machine, TF accelerator with 3.1 ROM and 4 to 8GB CFHD.
Your Amiga 500 system as it stands is a great example of an original classic. Why change it? Get a Towered A500/A600 or A1200 and expand the ass out of that instead! This way you have original classic and unique modern original classic together.