You shouldn't have a problem running the 5770's at 16x/4x. Here are two reviews showing how the 5870 and GTX480 are barely affected. The version of PCI-e does make a difference. What version of PCI-e is your motherboard?http://www.techpowerup.com/reviews/AMD/HD_5870_PCI-Express_Scaling/1.html
"...we will green-signal installing a third accelerator for some 3-way ATI CrossfireX action, or 2-way CrossfireX on entry-level Intel P55 motherboards with the second x16 slot electrically x4 (running in 1.0 mode)"http://hardocp.com/article/2010/08/25/gtx_480_sli_pcie_bandwidth_perf_x16x16_vs_x4x4/
"Even with all the data that GTX 480 SLI is pushing across the PCIe bus, x4/x4 is NOT a bottleneck in a single display setup at 2560x1600 with AA enabled."