I have decided to upgrade my Amiga 2000 PAL REV 4.5 for get 1MB of chip ram. I also took advantage of this update to install a new battery for the system clock. Amiga 2000 PAL REV 4.5 Fat Agnus 8372A Installation:

• Install a new Fat Agnus 8372A or 8375 (8375 R1)
• Look for jumper J101 beside the power connector to the motherboard. Change it to the opposite position. (photo)
• Look for trace J500 at the back of the machine near the Cia’s (8520′s). It’s a jumper trace that you will have to cut with a razor blade. (photo)
• Look for trace J102 near the Fat Agnus (8372A). It’s a jumper trace that you will have to cut with a razor blade. (photo)

source: amiga-hardware.com amigahardware.mariomisic.de amiga.org

from C64scene homepage:

Tomi Malinen is currently developing a dual SID board, although there are other solutions available for your C64 this one is interesting because of the ability to use either 6581 or 8580 as SID #2 no matter the board revision.

This means that you can play games with 6581 sound from the first channel and 8580 sound from the second.

Other features of the dual SID board are:

• Small board, only about 40mm x 50mm.
• SID #2 address jumper selectable between D400, D420, D500, DE00, DF00.
• MSSIAH-compatibility at DE00.
• Full game compatibility with double mono setting at D400.

Installation requires soldering, two wires attach to the cartridge port and two have to be soldered to the Kernal IC. When the SID #2 is mapped to D400, D420, D500 the wires to the cartridge port are not used.

Photo of both protypes installed:

source: c64scene.com lemon64.com