Especially since Diablo 3's release date hasn't even been finalized, it might be hard to figure out what their requirements would be.
However, that comp you've got there has seen a good number of years. At the very least, you should also be looking at a new CPU. They are still producing LGA775 boards, so it might be possible to just buy one and plug it in. This would save you a few dollars. Bear in mind that you'd probably save about US$200 not buying a motherboard and RAM to go with it.
However, buying a good GPU now, and plugging it into a new system in the foreseeable future (end of year?) is something you can do. Knowing the CPU is a bottleneck, you can still enjoy your games, and if its slowing down, you know what you'd need to upgrade.