What is the maximum rate at which two PCIe 1.0 device can communicate with each other?
PCI Express version 1.x operates a line rate of 2.5Gb/sec per lane. The actual maximum data rate is slightly less than 2Gb/sec per lane due to the fact the line rate includes 8b/10b encoding and the protocol overhead at the data link layer and the transaction layer.

The actual transfer rate is application dependent and depends on a number of factors (maximum payload size, how often flow control updates are issued, for example. If you wish to know more on that, I suggest inhaling the entire specification and reading some datasheets).

PCI Express can aggregate lanes in multiples of powers of 2, so you can have links that are 1, 2, 4, 8, 16 or 32 lanes wide (assuming they are available). 

Pro tip: PCI Express devices are required to operate at the design width (the number of lanes available, usually) and at a single lane. An 8 lane device is not required to link up at 4 lanes or 2 lanes. The requirement is 8 lanes (in this case) or 1 lane, so devices that have different link widths may still communicate.

So the answer is that any two PCI express V1.x devices can communicate at slightly less than 2Gb/sec per lane assuming they are in pre-fetchable space. A 4 lane interface would have a data rate of just under 8Gb/sec.

PJS