I'd try moving the card from one slot to the next just to see if perhaps you have a bad PCIe slot.
Also going back to the "low end-ness" of the 7300 and the 75W power deal....
I think a lot of the forum members here do want to have a complete understanding of how all the widgets in the computer work, but a lot of times things become a lot more "black and white" when there's a new technology comes out that the newest Macs don't have. By black and white I mean, we can't possibly get along with what we have now we must have the new thing (no offense to the anyone, but see the thread where the guy got a new Mac Pro from his girlfriend).
The slot itself isn't limiting what cards Apple can use in any way. They can just connect an additional power line to the card from the power supply. PCIe 2.0 does increase the maximum power supplied (by defining a wider trace for more current capacity, this is something that can be done with PCIe 1.0 mind you like DFI did with it's Socket AM2 Infinity NF4 board) but thats about all it's good for at the moment. Of course, to make it more "Apple-esque" they'd probably just tag on another power connector on the motherboard as they did in the ADC days.
Low end cards do tend to be low power, but thats mostly because of how cut down they are compared to the high end cards of the generation. They also tend to be the testbed for die shrinks so the "low end" card will often times have the more advanced process. The Radeon HD 2600Pro/XT for example was the testbed for the 65nm process that was then used to make the HD 2900Pro. Die shrinks create cooler chips. As a result the 2600Pro draws around 26W.
For Apple, the reasons they use the 7300 could be:
A. they have a contract for X number of units or for X period of time.
B. they're going to use new cards as an incentive to buy the new Mac Pros coming out.
C. they're dirt cheap these days (not that they were really ever expensive).
My bet is on B & C.
The 7300 fits the crowd they market the machine to (professionals, hence Mac Pro). They don't mind fleecing you for a better card at all. The Mac isn't a gaming platform of choice per se so they can get away with it.
Of course, this is no different from what any company does. Any PC website/user that lambastes Apple for doing so should take a look at what Dell does with it's base XPS "gaming" Deskops come with Radeon HD 2400s and Intel Integrated Graphics. Same as in the "lower end" Inspirons.
