The transfer speed through the PCI-e generation 2 or newer bus can be sufficient to handle USB3.
PCIe v2 has a 5GT/s (giga-threads - bandwidth) standard the same as USB3.
The driver issue is basically the problem with all drivers: well written drivers work and play nice with Windows 7, poorly written drivers not so much. I can't imagine a product currently sold that would not have a Windows 7 driver.
But using a well respected brand and checking reviews should point you towards the products that work well with Windows 7.
One feature to look for is auxillary power (power connection from your power supply). It would be best to not be drawing operating and recharging power for drives and devices through the PCI-e bus.