Ah I knew the ports were backwards compatible but didn't realize the cards were too, so I did some research:
PCI Express 2.1 supports a large proportion of the management, support, and troubleshooting systems planned for full implementation in PCI Express 3.0. However, the speed is the same as PCI Express 2.0.
--- Unfortunately, the increase in power from the slot breaks backward compatibility between PCI Express 2.1 cards and some older motherboards with 1.0/1.0a, ---
but most motherboards with PCI Express 1.1 connectors are provided with a BIOS update by their manufacturers through utilities to support backward compatibility of cards with PCIe 2.1.
So I'm assuming since the manual doesn't say pci express 1.1, you should stay away from pci express 2.1 cards. And like echo says, your case is pretty small, so anything pci express 2.0 low profile should fit, just be sure to check the dimensions of both.
Give this a look, it's pci express 3.0x16 so it'll go in your x16 slot, it's a low profile card, but I can't find its dimensions:
Newegg.com - VisionTek 900574 Radeon HD 7750 1GB PCI Express 3.0 x16 CrossFireX Support Low Profile Ready Video Card
Do you think this would fit echo?