i'm running my Decklink HD Pro in slot 3 with the atto UL4D in slot 4 and HD has worked like a champ
G5 dual 2.5, 4.5 gigs ram:
SYSTEM A - Mac OS X.3.8, FCP 4.5HD, QT 6.5.2, Decklink HD Pro, Blackmagic 4.8 driver, Huge U320RX 1.2TB, ATTO UL4D with 3.3.0 driver
SYSTEM B - Mac OS X.4.1, FCP 5, QT 7.0.1, Decklink HD Pro, Blackmagic 5.0 driver, Huge U320
It will work in slot 3 as long as you don't put anything slower than 100MHz into slot 2. The trick to getting the card into slot 4 is to gently lift the plastic transistor (or whatever it is) that sits on the upper right corner of the PCI-X slot (I like to use a long flathead screwdriver) once you lift this out of the way the card slides right into place.
It's probably best to put the card in slot 4, all our BMD systems (4 of them) have the card in slot 4 and work fine.
If you're having to "fight" to get it in, it's most likely that the metal shielding on the mac is catching the lip of the card as you insert it. The pressure to push past the black plastic thing is slight, but if it gets caught on the shielding, no amount of force will get it in.
On some models of G5, it is pretty easy to fit a DeckLink HD in slot 4 but I've found it much more difficult on some newer models of G5 and so have just used a 100 MHz slot rather than place too much strain on the card or computer. You might also like to refer to the technical note named How to Fit a DeckLink 2 or HD in the Top Slot of a G5 which provides some hints as to how to do this.
The DeckLink HD series cards use BNC connectors as we found that alternative forms of connections were inadquate for meeting the SMPTE specification for HD-SDI. Unfortunately that means that the bottom connector can clash with a black plastic plug on the motherboard of the G5 and we've asked Apple if they would please move that plug out of the way in future revisions of the G5.
It is partly due to the space restrictions imposed by PCI slots that we will soon release the new Multibridge models using PCI Express. These models will have a tiny adapter card in the PCI Express slot and this will connect to the external Multibridge box via a PCI Express cable. This allows us to add as many connectors as we want without having to worry about running in to space restrictions in new models of computers.