Read this similar case:
Taken into consideration that the R7 260X is a rebranded version of HD 7790:
I don't think you could make that hybrid crossfire without problems or performance issues.
A more experienced user should correct me over this matter and give you a more appropriate advice.
gigabyte X58A-UD3R rev.2, core i7 975 3.9ghz (stock voltage), windows 8.1 64-bit, corsair 12gb DDR3 1600mhz 7-7-7-20 RAM, xfx HD 5970 + gigabyte HD 5970 (950mhz/ 1100mhz), coolermaster ultimate 900w PSU.
amd catalyst 14.12 Omega Driver