The DP 2.0 will be DDR ram with a max of 8GB. The DC 2.0 is DDR2 ram with a max of 16gb. If you have the opportunity to get a DC 2.0 cheap, I would still. Mine benches the same performance as my 2.16 C2D Macbook and has 0 performance issues other then HD video play back on high res in quick time (VLC doesn't have any problems).
The G5 would be a huge performance upgrade over the eMac you have, and sometimes outweighs the cheaper Mini thanks to 2 internal disks and expansion slots. The real question is Snow Leopard, and do you want to run it. If so, don't bother with the G5.