Stratix GX transceiver usage.

REFCLK range is detailed in Table 1–18. Stratix IV GX Transceiver Specification as 50-637.5MHz

resolution of data increment is not an issue. if you need an odd data rate for instance 1234.56Mbs, you simply need to choose a REFCLK that combined with the PLL multiplication settings will give you that figure. Open up the ALTGX Megawizard and try it.

I would not assume for your design that the transceivers will go higher than 8.5GBs. They may well do but its best to stick with what is specified.

x4 3.750Gbs should be easily achievable.