That Thunder IV is a very nice video card. I have the earlier "red nubus card" Radius/Apple 24AC and it's a good card, faster than a SuperMac Spectrum 24 in my experience. I imagine the Thunder IV to be even faster.
The IIfx's fast CPU will be able to pump data to the Thunder IV in a sufficient manner although that card is truly ideal for a Quadra or PPC machine.
Check the capacitors on your IIfx's motherboard! I have two motherboards for the IIfx and one of them is starting to have cap leakage. Apple oddly made the boards with 70% SMD tantalum with 2 SMD electrolytic cans over by the RAM and PRAM batteries. The board has pads for both types of caps, so Apple could make them with any mix of caps. The axial electrolytic caps seem to be fine.
Edited by IIfx, 02 November 2017 - 12:55 AM.