Depends on the motherboard revision: versions prior to 4.0 are terrible to add a new Agnus.
for newer boards, just a Megachip, MiniMegiChip or Megichip. Those are boards with the "updated" 8375 Agnus and one more Megabyte on it. Expect to pay around US$100 or more for one of those babies.
If your board is 6.0 or newer, there is a hack: piggybacking the same memory chips over the original ones, bending one of the address pins, adding a demux inside the Amiga and more important: adding a 8372AB Agnus (the one from the A3000).
The last Agnus is the only one compatible with the A2000.
