I want to second Dan's recommendations. Heres a bit of added info:
For the Z600 the BIOS is like for the xw6600.... they include access to fan speeds and important temperatures straight from BIOS itself. That is under the first "tab" of BIOS. I have seen some programs that you can run from the operating system that probe for fan speeds and temperatures but give incorrect information when compared to the BIOS approach. So, make sure to double check your numbers with those reported by the HP BIOS.
Second, the fan you circled is attached to the "southbridge" of the motherboard, and some workstations don't even have a fan for the heatsink there (the xw6400, for example). I've worked to optimize my workstation performance and reduce noise levels (medical environment), and thus have experimented with that fan both on the Z620, Z600 and the xw6600.
Noctua has in-line 4-wire fan speed reducers that fit perfectly for this fan/motherboard connector... they're about 4" long. I have their Low Noise Adapter, and their Ultra Low Noise Adapter. I also sent off to Noctua in Austria and got a few of their even higher resistor value adapters (I call it Ultra Ultra), and always run at least one of the Ultra Low Noise Adapters to reduce fan noise from that little high rpm fan. I've watched the temps for each value (stock, LNA, ULNA, and even the UULNA) and don't see heat issues from doing this.
I personally feel comfortable recommending that you plug in a LNA, which is easy to find on Amazon. So, don't worry about the temps for that heatsink/fan if you're running stock speeds (HP already programs in some speed reduction via the PWM features of that fan versus running it at 12VDC without PWM). Here's some results:
HP Z600 "chipset" fan RPM at stock with HP PWM from motherboard: 2530
Same, with ULNA adapter added in (82 Ohm resistor): 1925
Same, with the UULNA adapter added instead (100 Ohm resistor): 1800
I went straight to a ULNA and then UULNA on this build first so I don't have the RPM value for a Noctua LNA, and am happy with the noise level and cooling performance using the Noctua 4-wire adapter with the 100 Ohm resistor). We're not pushing these workstations hard, so if you're doing processor-intensive work and want to drop the RPMs this way just watch via BIOS for how things might heat up above what you'd see normally. You'll get accurate readings with a quick reboot into BIOS right after you push the workstation.... things don't cool off instantly, and if you find a program that gives you accurate readings from the OS then you can monitor real time.
The motherboard header and stock chipset fan plug is the conventional PWM type of morphology, as are the ends on the Noctua adapters, so it is pretty much plug and play.