I'm using the Quadro 2000, Quadro 2000D, and the Quadro K2000D in our xw6600 and xw8600 builds. Those work fine there, and in our Z620 builds.
Our workhorse workstations for medical imaging still use the Quadro FX1800 in the top PCIe x16 slot and the FX3700 in the lower PCIe x16 slot, but that use does not demand the 3-D power that you seek.
So, my recommendation is to get a Quadro 2000 used off eBay (I like the older silver version with the squared off rear end, rather than the black rounded rear end one. Silver gets you the larger quieter fan). That won't cost you much, and it may suffice. If you want to step up to faster the K2000 is nice but more cost for not a huge increase in performance.