The Z820 has LSI SAS chipset on its motherboard, unlike the Z620. Here is what I would do:
1. Make sure you have the latest BIOS installed... it is safest to update BIOS from within BIOS, and that has been posted about here multiple times.
2. I'm going to assume that you'll not ever be using SAS drives, and with the advent of better faster SSDs I bet that is correct. Therefore you can turn off that device via BIOS (in the security section). That way it becomes invisible to the OS both during OS use and during install of the OS from the installer DVD. That single act can save you significant issues.... it makes the machine look like it literally does not have that chipset on the motherboard. This is a very clean solution.
3. Some hard drives come from odd origins. Maybe Mac OS; maybe Linux; maybe a server OS and as such in deep areas of their boot sectors strange things may remain rendering Window OS install haunted by the leftovers. Unless I get a new from factory HDD or SSD I always do a very low level reformat, then a long-type NTFS format, and during the OS install I actually delete that single large partition and let the OS create its small "system reserved" partition, plus the normal large OS/programs/data partition. The tool to do the low level reformat I always use, and have used for years, is the free downloadable DBAN iso, from which you burn a bootable CD. For large drives it usually needs to run over night. For huge drives it can take a couple of days. This has saved many a used drive for me, and the net result is the used drive becomes "raw" again.
4. When you load W7 make sure you have set your SATA emulation in BIOS to RAID + AHCI (and in some HP workstations that is not a listed option.... you get the same thing by setting the option to RAID. This is strongly recommended by Intel and HP, and if you set that to IDE then you'll have some added mess to clean up before your modern drives will work properly.
5. Too bad on the 32-bit issue...... And, what about getting a SSD instead of that pokey HDD? At least consider that, and my favorite is Intel 320 series 300GB used off eBay. Take a look..... the Intel "Toolbox" software works great, and lets you update its firmware easily if needed.