I have not used a Quadro FX3400 but I can tell you that I very much like the FX3700 and we're using a bunch of those under Windows 10 Pro 64-bit Anniversary Update. Price is good off eBay, used. I did check, however, and the Quadro FX3400 is supported still by nVidia with drivers under W10Pro64AU.
I don't know if there is a HP card firmware updater for the FX3400, but there is for the 3700, and I always firmware update to the HP firmware, even if it is a nVidia Quadro card from Dell/Lenovo, etc. Those HP firmware updaters work on other brands of the same card.
Go to nVidia.com/ Drivers/ All Drivers/ Quadro/ FX3400/ subselect to OS W1064 Anniv Update/ Get the ODE ones only/ and install those. Should get you going.
Personally, I'd step up to the FX3700 to keep your tuned xw8400 balanced. With the W7 Windows Experience Index I'd bet your current video card will stand out as the weak link in the system now. Ideally you'd have the snap of the video card to match the other components you have built up. A Quadro 2000 would be an even nicer step up if you can find a good buy, but that is a luxury. I have two home main testbeds for best performance/cost. One is a xw6400 with a FX1800 and a FX3700, and the other is a xw6600 with two Quadro 2000's (D and regular version). I can hardly tell the difference. Then, there's the Z400/Z600/Z620, etc. But those other two are for one purpose.... proof of biggest bang for the buck.
I have a very compulsive method for clearing all nVidia drivers down to zero, and rebuilding back up to proper drivers if you get stuck.
The xw6400/ xw8400 with 16GB RAM (4x4 for the xw6400) run amazingly fast and stable with the processors I believe you are using, and that Intel 320 SSD.
Have you read my posts on the need for the special xw workstation storage controller ESB2 drivers to make sure that SSD TRIM not only is enabled but functions? The ones that MS chose for the default W10 load are incorrect, and enable but do not allow TRIM automatic function. You want to update to the correct ones so the OS does that for you. If you stick with the current ones then run TRIM from the Intel ToolBox once per month.
That post is HERE.
Scott