10G on twisted pair isn't too bad either. Used enterprise NICs are cheap on eBay. New low cost cards are available, too (still costs more than used gear, but if you want lower power or multigig, it's worth a look). Switches aren't so cheap, but IMHO, 10g-baseT is comparable to sfp+ and a transceiver on each end.

Obviously, if something is wrong with your existing cabling, 10g-base-t is probably not going to work either. 10G is probably the limit for twisted pair, but I think it'll hold up as a reasonable speed for a long time.