There are no PCI-X graphics cards - you're confusing PCI-X with PCIe, aren't you?
Note that many people are making the argument that PCIe is needed for *availability*, not performance. There's a fast shift to PCIe for higher end cards, and not as much new for AGP.
Staying with AGP will mean less choice and higher prices.
And, BTW, lots of 128MiB and 256MiB PCIe graphics cards for less than $40 at
http://www2.newegg.com/Product/Prod...tion=pci+express&Order=price&CompareItemList= ....
Apple has one PCI-X 133 MHz slot and two PCI-X 100 MHz slots in the high end, 3 PCI 33MHz/64-bit in the low end G5. So, you're comparing two technologies that Apple doesn't support (PCIe and PCI-X 2.0).
Note that the real numbers are:
Table 1: Bandwidth of PCI, PCI-X, and AGP Buses
Code:
Bus 32-Bit Transfer Rate 64-Bit Transfer Rate
33-MHz PCI 133 MB/sec 266 MB/sec
66-MHz PCI 266 MB/sec 532 MB/sec
100-MHz PCI-X Not applicable 800 MB/sec
133-MHz PCI-X Not applicable 1 GB/sec
[COLOR=DarkRed]266-MHz PCI-X2 Not applicable 2 GB/sec
533-MHz PCI-X2 Not applicable 4 GB/sec[/COLOR]
AGP8X 2.1 GB/sec Not applicable
Table 2: PCI Express Bandwidth
Code:
PCI Express Encoded Data Rate Unencoded Data Rate
x1 5 Gbps 4 Gbps (500 MB/sec)
x4 20 Gbps 16 Gbps (2 GB/sec)
x8 40 Gbps 32 Gbps (4 GB/sec)
x16 80 Gbps 64 Gbps (8 GB/sec)
[COLOR=DarkRed]x32 160 Gbps 128 Gbps (16 GB/sec)[/color]
(Tables from
here, PCI-X 2 lines and x32 PCIe added.)
On the other hand, my Dell PW670 has 3 PCI-X 100 MHz slots, two PCI 32/33 slots, a PCI Express x4 slot, and an PCI Express x16 slote (with 150 watts of power available) in a system the size of the PM G5. (It also support 4 internal hard drives as well as two internal opticals.)