I've read benchmarks
showing that up to 2560x1600, there is no performance difference between x8x8 and x16x16 for dual card, it stated showing a ~5% difference at 5760x1200 (three 1920x1200 side by side). This applies to SLI, not sure if it would be the same for XFire.
Currently you are correct. Hell the AGP 8x bus was not nearly saturated when the move to PCIe was happening. But if you throw a HD5870 on a AGP bus I am sure it will bottleneck it like crazy.
My only point is that the X38 chipset will allow for the system to live longer than the P45 will plus since it is a performance chipset will probably overclock better as well.
That or he should just go for the X48 chipset mobo.