Let me guess, she's got a Dell, right? In any case, it's a damn shame that a P4 2.8 has to be saddled with a PCI video card, but if she doesn't want a new case, what can you do?
I've been down this road a couple of times, and for the types of games you listed, the 9100 PCI will definitely be the better card. In a fair contest (given a 128-bit 5700LE and 5600), a normal 5700LE is comparable in speed to a normal 5600. In the PCI world, the 9100 is similar in performance to a 128-bit 5600 PCI. Given that the 5700LE is a 64-bit card, the 9100 should eat it for lunch.
To answer your question, going from a 128-bit card to an identical card with a 64-bit interface cuts the performance down almost by half.