I am working on a Z600 project right now, so this is timely I'll try to be brief, and you will need to use the search box on this page upper left area, which searches this forum only.
1. First step is to see if you have an original motherboard or the later "revision 1" motherboard. Best way to tell is if you look up the boot block date in BIOS, under the first tab. The boot block date tells you if you have original or revision 1 most easily.
2. Assuming it is the better later motherboard, then you need to find the latest Z600 QuickSpecs.... its in the 50s, maybe 51 or 58. From that you can look at the HP supported processors. Your memory should be the faster 1333 type but most of those processors listed there run slower than that. 3 run at the faster speed which will let your memory run faster too. You can choose from those 3 for the best heavy lifting. Note that the fastest quad core is faster than the fastest hexacore. So, if you want raw speed I'd bet the quad core would serve you well. If you want 12 rather than 8 total cores then go with the fastest hexacore.
3. You'll want 2 processors, and you'll want to match to the heatsink/fan you already have.
4. I believe you don't need a "performance" heatsink for 95 watt processor.... the motherboard wants one for processors that are over 95 watts, from my reading. There is a way to spoof a motherboard into thinking it has a performance heatsink attached when it only is a mainstream, by the way.
5. SSDs.... I have been very pleased with the Intel 320 series 300GB SSD that you can get off eBay. We use a lot of those.... rock solid, and the Intel Toolbox is great for tuning it. That workstation is a SATA gen II one, and HP warns against using SATA gen III drives in those. That particular Intel SSD is a SATA gen II drive with enterprise features, and has served us very well. Even used off eBay they almost always have had 100% life left in them. I update their firmware as needed via the Intel Toolbox.
6. We use W7Pro64 mostly, but I have been pleased with the W10Pro64 OS. You can upgrade to W10Pro64 for free at this time, so I have been "W10 activating" all my xw and Z workstations and having that ready for the future, but still mainly running them with W7Pro64. Once your box is W10 activated you can install W10 anytime later. You certainly want a 64 bit OS.
7. You need a second processor to take advantage of the second set of 3 memory slots..... ideally you'll run with 6 x 1GB, 6 x 2GB, or 6 x 4GB. The sweet spot for $/GB is with the 2GB sticks. Having them all the same size gets you a speed benefit too.... Look at official HP sticks for that on eBay and you also can find the same codes from other vendors such as Dell. I consider the HP sticks the best because they have been "binned" for quality.
8. HP just came out with a new BIOS for the Z600. I have gathered best HP driver sets for W7Pro64 from later HP workstation sources and they load fine on the Z600, but for W10 I've been using what comes from the autoload from MS.
9. I like to add in a quality eSATA backplane adapter to the bottom opening for fast backup and Acronis image captures. I also like to add one of the TI-based HP USB3 cards into the top PCIe slot, and am working on a nice use of that with the spare front 5.25" OD bay for both front and rear access to USB3. I'll post pics of that and in the xw6600 when I get the final parts.