Is the Is VisionTek Radeon 5450 2GB Graphics Card - 900861, Black/Red compatible with my Quicksilver G4 867?

No. A QuickSilver G4 uses an AGP card; the card you are looking at is PCIe. Specs:

https://www.visiontek.com/products/visiontek-radeon™-5450-2gb-ddr3-dvi-i-hdmi-vga

Also any card not specifically marketed "for Mac" won't work unless someone "flashes" (hacks) the ROM to change it to Mac-compatible.

Good resource for G4 owners here: http://www.jcsenterprises.com/Japamacs_Page/Hardware.html

I still refer you to the "All Things PPC" link I posted for recommendations. My G4 was an MDD and I never replaced its card, a Radeon 9000 64MB.

There is some cross-over of slot compatibility between PCI, AGP, and PCIe but all the images of QS slots make it appear those slots are incompatible with the connector on the PCIe card you mention. The larger issue is your MUST find a card built for Mac. Generic PC cards must be flashed and that is not a trivial matter.

As drivers for old cards are hard to find any more. I would search used/pulled sited for another nVidea Mac card. The vendor I usually recommend seems devoid of all old pulled cards, but you coudl try an auction site. These cards were factory-instelld in the 867mhz Quicksilver, for the MacTracker databate:

So finding one of those sold as FOR MAC is my best recommendation given the driver situation.

What card came with the Quicksilver?

Look at the resource link I provided. In addition to specific card models that are Mac friendly, it has info on flashing cards sold for Windows computers.

I do not believe you will find any new cards out there unless they are "new old stock" and I found those on eBay.