Mic-E,
Yes, 100C is well over the Intel rating of 75C for the X5550.
I do not have experience with the z600, having two z420 and a z620, but looking at images of z600's and considering the way the z620 is configured for cooling- quite careful clearances and special position of nearby compoents and air path, my suggestion is to first consider buying new or used HP CPU coolers on Ebay or similar. For example:
NEW HP CPU Heatsink with Fan for Z400 Z600 Z800 Workstations 463990-001> $39.99
The dimensions are crtitical. The CPU cooler towards the rear panel has a very limited clearance to a tooless panel that appears to be the fan/shroud for memory cooling.
Otherwise, as the z600 is Socket LGA1366, it's possible to consider aftermarket fan .heatsinks or liquid cooling solutions, but take great care in selection as regarding dimensions. Personally, I'd use the standard HP coolers and install carefully using a good thermal paste such as Arctic Silver.
If the z600 has a similar BIOS setup to the z620, thre is a "Thermal" control that allows the fan idel speed to be advanced. My z620 with two 8-core C{U's was running at about 45-48C at idle. I advnaced the fan setiing 3 asterisks of the 6 possible and the ptemperatures are now 35C-39C.
I like LGA1366 very much and have two systems using it. If you need a bit more fizz in your z600, I can highly recommend the X5677 4-core @ 3.47 /3.73Ghz and X5680 6-core @ 3.33 /3.6GHz. The X5550 has a Passmark Single Thread Mark of 1250 whereas the X5677 = 1516. those cost about $60 in the US these days (9/16)
Cheers,
BambiBoomZ
z420 > Xeon E5- 1660 v2 / 32GB / Quadro K4200 / Samsung SM951 (256GB) + Intel 730 (480GB) + WD Black 1TB / Windows 7 Professional 64-bit > 2X Dell Ultrasharp U2715H
z620 > 2 X E5-2690 / 64GB / Quadro K2200 / HP Z Turbo Drive (256GB) + Seagate Constellation ES.3 1TB / Windows 7 Professional 64-bit > HP 2711x
z420> Xeon E5-1920 / 24GB / Quadro 4000 / Samsung 840 250GB + WD RE 1TB > Win 7 Professional 64-bit > 2X Dell 24