They're AMD FirePro cards.
And no, you can't perform internal upgrades, but the computer is ridiculously powerful, and thunderbolt 2 ports on the outside allow for new external upgrade options... already they got external PCIe graphics cards working with the first version of Thunderbolt, albeit slowly, I think each thunderbolt 2 port gets like PCIe 8x bandwidth. It's possible that the Xeon processors inside may not be overclockable, so... the aluminum core (very big heatsink!!) probably was tested to be over-adequate to manage temperatures, and the point of that huge heatsink and huge fan is low rpms high efficiency cooling, so it'll probably stay cool enough while staying quieter (with lower maintenance?) than a liquid cooler pump and system.
Pretty sure you don't have an interest in actually using OS X anyway... Kind of like I don't want to use Windows, it's just that for some games I HAVE to (hopefully Steambox removes that requirement?), most of the productivity and creativity apps I want to use I own for OS X. You'd probably only use OS X out of curiosity (which is fine, but I don't think you have a need for what Mac offers).
All that said, the power of it is waaaaaay overkill for my needs... it would just be the second kind of cool for me to own :D
I probably should just get a 27" iMac instead, dual boot it with Steam or Windows.
I know what GPUs are being used on those cards, but honestly, do those look like cards that would fit in a typical PC case? They're not exactly off-the-shelf hardware, especially given the cooling system used. At least with the old Mac Pro and the Power Macs that preceded them, you could use whatever PCI/AGP/PCI-Express cards you wanted. And while there are external Thunderbolt PCI-Express bays, I'd be left feeling like the internal FirePro GPUs are just dead weight if I needed the external PCIe bays for graphics upgrade.
Keep in mind that today's "powerful" is tomorrow's "hopelessly obsolete", hence my being in favor of upgradability. The Apple answer to that question seems to be "buy a new computer"...though they're honestly not targeting my sort anyway, the sort of people who would change graphics cards like underwear if budget permitted it.
As for liquid cooling being noisy, it all depends on what parts you get. A Laing D5 (which most people know as the Swiftech MCP655 and all other sorts of rebrands) mounted properly is almost silent, and you can get radiators with low fin density, tuned to work best with slower, quieter fans that don't make much noise. It's still noisier than a completely passive-cooled setup devoid of fans, but at least in my experience, pump noise is generally drowned out by fan noise.
Thus, I don't believe Apple's "heatsink core" approach is necessarily cooler and quieter...but it IS inherently lower-maintenance in that you don't have to top off a loop with distilled water every now and then.
Indeed, OS X is nothing more than a curiosity for me. For that matter, the Mac I want most is a Mirrored Drive Door Power Mac G4 set up to tri-boot OS 9.2.2, OS X 10.4 Tiger and OS X 10.5 Leopard, since that covers most of the Mac software base I care about: games that haven't found their way to Windows for some reason. Unfortunately, said Macs still command a bit of a premium, like $300 or so last time I checked.
Of course, when I grew up on IBM-compatibles all my life, it's only natural that I don't feel as tied to Apple. Your case is different; you've already got a lot of Mac software. (Ideally, software should be "buy one copy, get ALL the versions", but it very rarely works out that way...)
In other news, UPS delayed that MMX300 delivery by a day. First impressions will have to wait.