The limitation isn't the NIC, it's the processor (and possibly storage). The 10 and 40 Gigabit NICs exist but processing them on a conventional PC is hard.

It is, just about, possible to perform actions on every packet in a 10Gb stream on an x86 machine. You have to use a userspace stack, handle packets across multiple cores, and be VERY careful with what you are doing so you don't do cache misses. At 10Gb/s you're talking only a few hundred clock cycles per packet - anything that doesn't work as planned causes massive backlog.

Now try serving (dynamic) HTTP to that.