Does PCI x16 Riser = Less Performance?

I'm in the process of finishing a super slim custom HTPC case. It will only have enough height, about 2.5in-3in, so that the a slim cpu heatsink can fit in it. The only problem I'm having is that i forgot my motherboard LGA 775 doesn't have onboard video. I have a 3870 but it's impossible to fit into that case unless i lay it flat. To do that I must buy a flexible PCI x16 riser. I'm not sure buy i think that adding a riser card can lower performance and have other issues. I really don't wanna invest in another mobo because LGA 775 is dead.

The Needed riser will have to be 3in. Will a PCI x16 riser have performance problems? What's the longest riser i can use before i run into problems?

Here's the flexible riser card:
http://www.orbitmicro.com/global/pe-flex16-p-759.html

-And Yes i do have a PSU that will fit in the case and power the card.