The Elegant Way!
Have you ever thought about the fact that you don't necessarily need the VGA loop cable? As I've described before it's the best to keep the cable length as short as possible. The solution is quite simple, all you need is a monitor with at least two inputs (BNC and D-SUB). Now connect your video card to the BNC input and the 3Dfx card to the SUB-D cable. You shouldn't chose the reversed way to connect the cables since the Windows picture quality will be the best with the BNC input, furthermore the signal quality is weaker with the 3Dfx card on the one hand and you won't even see any differences while playing games on the other hand. If a game activates the 3Dfx card, just switch over to the alternative input. With some monitors (e.g. Iiyama) it's even more elegant: Over an auto-option the monitor will switch to the active input automatically.