Yeah, because of fast disks and having 16gb of ram. If your G5 had a 2.5-inch 5400rpm disk and only 2gb of ram, the Mini would likely be faster.
There's also the whole "real world…
leosadv,
Specifically which Mac OS are you trying to use this card under, Mac OSX 10.3 or Mac OS 9.2.2? If it's 9.2.2, you could try and see if the card is compatible with the Or…