No, there's probably no way to access those connectors in the iMac. Are you sure you read the right specifications too, because I think the 2600 is too old for Blu-Ray. As I remember, it was introduced with the 4000 series earlier this year. As for surround sound, that'll just be handled on the sound card, either external or built into the motherboard/logic board, not through the video card. Even today's video cards don't handle sound even if they advertise it: they just pass it through an HDMI connection.
As for the sound card, it's probably integrated onto the motherboard. However, it does come with optical capabilities, so you'll just need a mini-Toslink to Toslink cable or adapter to use standard optical sound, which will carry a surround sound signal.
As for the RAM, it more than likely will work, but Apple certification is convenient, because it's guaranteed to work, other than possibly guessing the wrong type of RAM. For example, the Mac Pro uses (at the time of this writing) fully buffered DIMMs (FB-DIMM) with error correcting code features (ECC vs Non-ECC). Many RAM sticks you'll find in retail stores are standard DIMMs and non-ECC, which I believe will not work in the Mac Pro (a quick Google search will tell you). If you are able to find compatible memory sticks, they should work, but I don't know if they'll be supported by Apple should anything go wrong.
Does the iMac use the standard ATI 2600 Pro video card? On ATI's website it says that the card has ports for surround sound and blu-ray directly on the card. If so, is there a way I can open the iSight slot-loading 20" iMac that doesn't have the screws in the bottom without destroying the system so I can get to those ports?
Are there any ports that aren't available on the sound card? Is the sound card an integrated Intel chip or something else?
In a Mac Pro, does the hardware have to be from Apple? For instance can I go to Wal-Mart and get RAM that just matches the specs, regardless of whether it says Mac-certified or not?