I actually own one of those. I had it to 4.06Ghz easily with a QX9650 using Throttlestop 6.00.
You could tape mod the Q6600to 3GHz especially if it's a SLACR G0 stepping. Any of the faster Q9xxx are faster though.
It will run unlocked CPUs up to 130W. QX9650. No LGA771 Xeons. You can tweak with SetFSB. The X38 chipset has hidden support for 400FSB so there's the possibility of another pinmod from 333 to 400. But no Voltage control without TS and an unlocked CPU. T9303 is the good cooler. There's room for full size GPUs and aftermarket PSUs fit.
I never connected it to the Internet so no posted results. But here is the similar TP412 from The T3400 Workstation ( it has the 2nd 16X GPU slot installed). QX9650 @ 4.15Ghz.
http://www.userbenchmark.com/UserRun/3530803