It's true this is probably the only T7500 overclocking thread. But you mentioned the T3500 which is a piece of cake to overclock.
I will leave overclocking the T7500 to the OP here. But will share what I know otherwise.
Here's a link to a locked BIOS overclocking method. T3500 is popular there, and various LGA1366 CPUs are discussed. Since it requires unlocked CPUs dual CPU machines aren't supported except QX9775 systems.
And the Dell Workstation Owners Club.
Regarding question 1. You might look into SAS cards also. It's a SCSI format and is full duplex unlike SATA.
Question 2. The fastest dual CPU is the Westemere EP 4C/8T X5687 3.6GHZ 3.86 turbo. ( there are some 4.4GHz dual cores, but scary expensive ES chips.)
Question 3. Multiplier OC on unlocked CPUs is the way to go for single CPU workstations.
Question 4. I'll leave that to the OP here. But I will suggest studying the RAM speed characteristics and RAM types supported on that chipset.
A. RAM speed is determined by the memory controller on the CPU and it varies.
B. RAM speed also changes with number of modules per channel. 1 per is fastest.
C. RDIMMs can be used in the dual CPU machines, but not the T3500. They're very inexpensive compared to UDIMMs. They also come in larger capacities.
5. FWIW the T5500 MB interchanges with the T3500. Dual CPU support (on a riser like the T7500) RDIMMs can save enough to pay for the MB swap.
6. If your doing an FSB overclock on LGA1366 you should study The X58 Xeon Club because you will be changing RAM speed, and QPI link timings. You also won't have much control over these settings in the locked BIOS. If you can figure out where your OC is going to take you maybe you can buy some RAM that supports the needed speeds.
7. There is an unlocked dual CPU LGA1366 MB. The EVGA Classified SR2. It's a rare and expensive piece, but it will get you where you want to go.
The Voltage looks a little funky on that one, so here's another.