Here is some added information:
1. Your power supply is 320 watta..... nVidia recommends at least 400W for the GeForce GTX 1060 card you mention (which draws 120W max TDP and takes up 2 slots). That is the bad news.... but the good news is that video cards are truly the area where performance has gone way up and power draw has gone quite a bit down at the same time over the last 5 years. So, your performance/watt will be greatly improved if you buy a very recently released one that better matches your workstation.
My advice is to look at the latest nVidia cards that don't require an auxiliary power cord attached, or ones that do but are not so much above the power draw of the highest draw card HP has in its latest QuickSpec for your Z220. Having said that, the single PCIe Gen2 x16 video slot you have (second from the top slot) should provide about 75 watts up to the card and an auxiliary PCIe "6-pin" power cable can provide about 75W more. So, if you are not using much power elsewhere you might just get away with using a GTX 1060. I'd personally not do that, and you will still have such an improvement by taking 1 step down in power draw that it will be great anyway. You have other things that will be the limiting factors for your workstation (such as processor). You don't want to try to stuff 20 pounds of potatoes in a 10 pound bag.
2. Your Z220 is a "convertible minitower" (CMT) form factor type. As such it can accept both half and full height cards, assuming they have the full height metal backplane attached. Many HP workstations have a PCIe auxiliary 6-pin power plug available but coiled up at the rear of the power supply and zip tied there. Look for that and use it if you need rather than leaving it coiled up. It is a better way to go than using a power adapter to aggregate 2 molex or 2 SATA power plugs into 1 "6-pin" plug.
3. Search out Z220 CMT QuickSpecs, and the latest I found was version 22, HERE.
http://www8.hp.com/h20195/v2/GetPDF.aspx/c04123224.pdf That will give you up to date info on the wattage of cards HP approved:
FY944AA - ATI FirePro V3700 256MB PCIe Graphics Card TDP = 32W
FY946AA - NVIDIA Quadro FX1800 768MB PCIe Graphics Card TDP = 59W
FY947AA - ATI FirePro V5700 512MB PCIe Graphics Card TDP = 56W
NVIDIA Quadro K4000 3GB 80W max TDP.... this is getting you into the range where a 6-pin plug needs to be added.
That K4000 has a plug in site for a 6-pin PCIe supplemental power cable, so it clearly is OK to at least go up to that level of power, and HP is conservative on these things so you have wiggle room. I'd go nVidia GTX and EVGA as vendor personally. The exact card I'm not sure on, but one that has just been released recently, brand new. If there is an EVGA "superclocked" version for the one you choose I'd pay that extra charge. Most people cannot tell the difference between one of these running on a PCIe generation II slot versus a gen III slot. You only have 1 gen II slots, and the rest are gen 1, all quite fast.
4. Your Z200 is a SATA generation 2 rather than SATA3 workstation. Most users cannot perceive the speed difference between a SATA2 and a SATA3 drive running in a SATA3 workstation so don't worry about that issue. However, there have been some compatibility issues with using non-HP SATA3 HDDs and non-HP SATA3 SSD drives in HP SATA2 workstations.
I'd strongly advise that you run that Z220 off a SSD if you are not already doing that. The one I'd advise is an Intel 320 Series 300GB bought used off eBay. I've gotten a lot of these and almost all have had 100% of their life left when measured by the Intel ToolBox software, which is free, and allows easy update of the SSD firmware if needed, and also provides automatic fine tuning of the OS for the SSD as one of its options. When I get these I use a bootable CD burned from a free ISO download of a program called DBAN. This performes a low level reformat, which even zeros out the boot sectors. Then I boot into W7 and use the built-in Windows Disk Management utility to convert its raw state to a MBR partition, and then I do a NTFS long type format. Then I load in the OS from scratch, or I use Acronis to clone over an image from the source hard drive to SSD (which works fine assuming the HDD had been properly loaded initially). For example, it will have been properly done if it is the HDD that came with your Z200 from HP.
Hopefully you have your original HDD with its HP "Recover" partition unused on it, and can capture the HP W7Pro64 Restore Media from that (very valuable, and worth getting if you can). Capturing that as a dedicated USB is best.... use 8GB quality thumb drive. That installer then lets you boot from USB and it installs the HP OEM COA serial number automatically, which will be pre-activated.
There are lots of posts in here on how to convert a 2.5" form factor SSD into a 3.5" form factor drive so it can fit in your workstation easily. Slider screws will generally be available as spares inside the case for you to use.