The Dell T5500 & T7500's are a LOT quieter than the T7400 & 5400's. A lot of it has to do with the heat generated by the older Intel 5400 series CPUs that are 45nm architecture vs. the newer T5500 & 7500's use 32nm CPUs which in many cases (with hyperthreadding) can almost double the workload with the same amount of power.
The older Dells - P490's & P690's and some of the T3400's were LOUD but the T3500's & 7400/5400's got quieter and the 7500/5500 are even quieter still.
Kevin QC, take a look at the picture I posted. The two slots with the tabs are the two 75W x16 slots (Slot 2 & 4) and the two without the tabs (Slot 1 & 3) are PCIe x8 (speed) but still full x16 form factor (length).
All it means is that the PCIe slots 1 & 3 will provide a maximum of 25W and have a bus bandwidth and electrical capacity limited to x8 specifications vs. x16.
Here is a really good link/tutorial that will help explain a lot of this stuff (like speed vs. electrical vs. size):
http://www.tomshardware.com/reviews/pci-express-scaling-analysis,1572.html
Also, take a look at this picture:
http://www.tomshardware.com/reviews/crossfire-meets-pci-express,1761-3.html
Take a look at the labeling on the mobo, see how it says PCIe x16 (referring to the size of the slot or form-factor) but then in parentheses it says what electrical output (and bus speed reference) the slot will perform at.
x8 slots will give you 2gb/sec bandwidth whereas an x16 slot is double that performance at 4gb/sec. (reference link below)
http://www.tomshardware.com/reviews/pci-express-scaling-analysis,1572-2.html
It means that two of your cards will run @ x16 speed which is 4gb/second (or 8gb/sec full duplex) and one card will run at x8 speed which is 2.0gb/sec or 4.0 @ full duplex.
I can't find any tech specs but I doubt that video card can push more than 2gb/second - i could be wrong though. ATI has terrible technical specs so I really don't know.
A quick Google search showed guys running things like flight simulators or massive solidworks or CAD projects maxing out x4 and x8 slots with extremely high-end video cards. I think you are safe but to be honest without a decent tech specs document i have no idea.
Perhaps go on the ATI support forums and ask them?
Think of the x16 vs. x8 vs. x4 like a network card - there is 10/100 and 1000 mbits per second. or SATA version 1 was 1.5gb/sec, SATA2 is 3.0gb/sec and SATA 3 is 6.0gb/sec, its just speeds.
It looks like they list the x16 bus because of the length of the slot more than the speed requirements of the card - but hopefully someone else can confirm.