There weren't that many games that were ported from the Gamecube to the PC, but Resident Evil 4 was one of the games that was ... While it is not the best metric to compare PC hardware to console hardware (because you get additional performance fromoptimizing for specific hardware on a console which isn't possible on the PC) Resident Evil 4 required a mid-level Geforce FX (Geforce 5) to play on a PC and it didn't play better than the Gamecube version unless you had a high end Geforce FX or Geforce 6 based graphics card.
The reason why I bring this up is that if the XBox GPU (which was a modified Geforce 3) was so much better than the Flipper you would expect that any game ported from the Gamecube to the PC would only require a Geforce 3 or Geforce 4 graphics card ...
From what I recall, the strenght and weakness of the Flipper was that there was support in hardware for most of the texturing effects that developers would want to do ... When fully taken advantage of, the overhead from programmable pipelines (like the XBox's GPU) made it difficult to match the same quality level without being noticeably more powerful ... At the same time, few developers ever focused their efforts on the Gamecube so there were few games that ever took full advantage of the hardware that was there.
