A500 Upgrades

[Tenacious]

Ah yes, I see now.  You've got 4 but they are 4 x higher capacity than the 41256.
 
Just search for 424256, needs to be DIP package:-
 
http://www.ebay.co.uk/itm/D424256C-80-NEC-DIP-20-IC-/290458263493?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item43a0a95fc5#ht_63wt_1161
 
£5 each isnt too bad, but means £20 for the chips you need.
 
Then sockets:-
 
http://www.ebay.co.uk/itm/DIL-DIP-IC-Socket-6-8-14-16-18-20-Pin-Sockets-UK-1st-Class-Postage-/290767443526?pt=UK_BOI_Electrical_Components_Supplies_ET&var=&hash=item43b3171646#ht_1897wt_1161
 
You need 20 pin DIP sockets, which that seller has.
 
Simply solder the sockets in place, stick your chips in and I think you are away.  There might be a jumper or something, I will check in a min.

I might have missed something, but, I don't think this is a simple matter of adding more ram chips on the motherboard (even with resistors   ). Your Agnus addresses either 1 Meg of chip (preferred)  OR  512K of chip and the trapdoor mapped as 512K of slow ram.  It is my belief that any additional ram on the motherboard will have no refresh.  OTOH, you could add the 2 Meg Agnus .

The true fast ram boards mentioned earlier come with support circuitry for refresh.

Food for thought before you attempt this modification.