The Mac Radeon OEM card that Apple shipped in the DA PowerMac G4s works in the Cube and supports ADC. You'll have to swap the fence with one from a Cube video card. This is the card I have in my Cube.
I'd be interested in knowing what you end up using for an SSD in the Cube. I've been thinking about doing the same with my own Cube, but I'm not sure what combination of IDE-SATA adapter and drive carrier will fit.
Just an FYI if you weren't already aware. Apple had a battery recall for this model. I'd check the battery's serial number against this document just to make sure it's not one of the effected ones. The defective batteries were fairly common. Back in the day, I replaced roughly 20 of them out...
Those are pretty nice. That was the first enterprise-grade server I ever used Truly amazing engineering in the ease of access and the amount of redundant, hot swappable parts.
I found I pretty interesting InfoWorld review of this accelerator card. According to the review, properly configured, an SE with this card would outperform a Mac II.
Try calling Apple about it. They have been repairing PowerMac G5s with leaking cooling units out of warranty, even if you are not the original owner. You may have to talk to several levels of customer service at Apple, but keep stressing that its a serious safety issue, as the leaked coolant...