Your W3505 is already a 130W processor, so you have the Performance fan. Shift over to either the W3680 (search eBay for SLBV2, and you'll find some now at 93.50 USD). Or, if you want to go from 3.33 to 3.46 GHz speed spend the extra and get the W3690 for about 145.00 off eBay. That is a psychological thing.... you won't be able to tell the difference in the real world, and all the other main features are identical.
I have posted in here on adding a USB3.0 HP Texas Instruments card.... make sure it goes into a PCIe Gen II slot so bus speed will let it get to USB3.0 max speed. My post includes info on the HP 5.25 to 3.5 adapter that will let you place my favorite Akasa front interface into that and mount it below your optical drive in the spare bay up top. I included into on the correct Molex brand 4-wire peripheral to SATA power adapter that I get from Mouser.com to fit that card perfectly. You end up with 4 USB3 slots, 2 rear/2 front.
Here are some links on that process:
You have the SSD..... Gen II vs Gen III is not really an issue..... stick with plugging that straight into the SATA port 0 on the motherboard. Dump all SAS.
I've posted here on how to drop your boot times dramatically.... read up on that. Use the forum's search box above to find these things.
I added a 40x40x20mm Northbridge fan, 4-wire HP version, from a xw6600 motherboard. Mine is screwed atop the normal silver Northbridge heatsink fins, and there would be several ways to attach that. The fan blows up away from the motherboard pulling cooler air in through the heatsink from below. The same fan is used on the Z600 Northbridge. There is a spare motherboard standard PWM white header near there, and I added an in-line Noctua 4-wire LNA to drop its speed (which I do for all the HP Northbridge fans). That adds to the length overall and lets the fan plug reach the motherboard header. The RPM speeds now show up in the temp/rpm section of the Z400 BIOS as a memory cooling fan. So, the original header appears to have been intended for a memory cooler, but it is perfect for the Northbridge cooling. The speeds of either of those 2 processors would boost the heat at the Northbridge, and that was my solution (but probably that is not really needed). I don't know if HP ever did a similar things for some high end Z400s, but seeing that fan used on the xw6600 and Z600 inspired me to add that to this Z400. You can get those fans with the proper HP wiring by searching eBay, and from China where they were all made. That specifically is the Delta EFB0412HD -7R49, and that modifier at the end designates the HP 4-wire PWM controlled version.
You want all 6 memory slots filled by identical 2 or 4 GB sticks, with 1333 memory speeds to take best advantage of the HP memory engineering and the fact that they'll be able to run at that speed with those processors.
The HP PCIe supplemental power 6-wire cable provides significantly more power than the ATX 75W max standard..... I don't see an issue there for your video card needs, within reason. Your net power use will be going down rather than up, likely.