I would honestly get a DDC with the XSPC restop instead. I think it would be a cleaner install than putting another D5 and MCRES-Micro in there.

The EK block should work. You could use the SLI bridge, or some D-plugs and just run them in parallel. Make certain to check reference design though before purchasing blocks. I don't think you need to water cool the 9800 GT, what temps does it reach?

You really don't need to put the board on water, but if you did, just loop it in with CPU and a 360 rad, and you should be fine. As loong as you have some airflow or a fan pointed at board, it should be fine.