Let me rephrase that to avoid confusion:
1./ There are many versions of AGP. What I am refering to, and only refering to, is AGP version 3.0 also known as AGP 8x with a bandwidth of 2112MB/s. I will refer to AGP 3.0 (8x) as AGP for short. Any mainboard with a lower AGP specification is an old dinosaur and is not part of this discussion.
2./ PCIe16 has twice the "one way" bandwidth compared to AGP. PCIe16 can sustain 4000MB/s one-way.
3./ Because PCIe16 can read and write in both directions, sometimes PCIe16 is claimed to have 8000MB/s bandwidth that is nearly four times the bandwidth compared to AGP.
4./ However, since we "write" and dont "read" to GPU then this point is moot.
5./ Hence, from a GPU perspective, we can think of the PCIe16 as being twice as fast as AGP, 4000MB/s compared to 2112MB/s. Also, PCIe8 is about the same speed as AGP in graphics bandwidth terms, 2000MB/s compared to 2112MB/s.
6./ For interested parties, the bandwidth obtained through simultaneous read and write IS VERY INTERESTING for network cards. Basically a PCIe1 network card can operate at 2-4x the throughput of a PCI network card. And on a server, you can install network cards with multiple connections that run on PCIe4, which has 8-16x the throughput of a PCI card. PCIe also doesn't "steal" bandwidth from other slots. Whereas on PCI, this bandwidth is shared across all PCI slots.
For reference:<div align="center"><table width="60%" border="4">
<tr>
<td><div align="center">Graphics Standards</div></td>
<td><div align="center">Possible Bandwidth</td>
</tr>
<tr>
<td>
<div align="center">16-bit ISA</div>
</td>
<td>
<div align="center">16MB/s</div>
</td>
</tr>
<tr>
<td>
<div align="center">EISA</div>
</td>
<td>
<div align="center">32MB/s</div>
</td>
</tr>
<tr>
<td>
<div align="center">VLB</div>
</td>
<td>
<div align="center">132MB/s</div>
</td>
</tr>
<tr>
<td>
<div align="center">PCI</div>
</td>
<td>
<div align="center">132MB/s</div>
</td>
</tr>
<tr>
<td>
<div align="center">AGP 1x</div>
</td>
<td>
<div align="center">264MB/s</div>
</td>
</tr>
<tr>
<td>
<div align="center">AGP 2x</div>
</td>
<td>
<div align="center">528MB/s</div>
</td>
</tr>
<tr>
<td>
<div align="center">AGP 4x</div>
</td>
<td>
<div align="center">1056MB/s</div>
</td>
</tr>
<tr>
<td>
<div align="center">AGP 8x</div>
</td>
<td>
<div align="center">2112MB/s</div>
</td>
</tr>
<tr>
<td>
<div align="center">PCIe x1</div>
</td>
<td>
<div align="center">250 / 500MB/s
(Per direction / Both Directions)</div>
</td>
</tr>
<tr>
<td>
<div align="center">PCIe x2</div>
</td>
<td>
<div align="center">500 / 1000MB/s
(Per direction / Both Directions)</div>
</td>
</tr>
<tr>
<td>
<div align="center">PCIe x4</div>
</td>
<td>
<div align="center">1000 / 2000MB/s
(Per direction / Both Directions)</div>
</td>
</tr>
<tr>
<td>
<div align="center">PCIe x8</div>
</td>
<td>
<div align="center">2000 / 4000MB/s
(Per direction / Both Directions)</div>
</td>
</tr>
<tr>
<td>
<div align="center">PCIe x16</div>
</td>
<td>
<div align="center">4000 / 8000MB/s
(Per direction / Both Directions)</div>
</td>
</tr>
</table>
Now the main point is this. If AGP is as good as PCIe8 and is 2x the speed of PCIe4. But we know that PCIe4 and PCIe8 are good enough for SLI and crossfire, then we know AGP is also good enough and we will not hit a bandwidth problem on AGP.