Different 6500 line cards do have different architectures regarding bandwidth to/from line ports. Very possible one of the older line cards, especially one for "users", rather that for "servers", had 8 Gbps shared between 8 gig ports, but as Reza notes, a 6748 should have 20 Gbps shared between 24 ports. (NB: I believe Reza might be mistaken about all 67xx cards have dual 20 Gbps fabric connections. I recall the 6724 only has a single 20 Gbps connection.)
If you're also wondering about the impact of a CFC vs. DFC equipped 6700 line cards, it shouldn't make any difference to raw bandwidth capacity of the card, itself, but it can impact actual bandwidth capacity to/from the card as CFC cards all share the sup's capacity for forwarding while the DFC equipped cards perform forwarding locally on the card. Additionally, what else is in the chassis impacts bandwidth capacity. For example, a chassis with only 6500 fabric cards, allows double the MPPS rate on a sup720 and the MPPS rate is not impacted by frame size. Conversely a 6700 line card passing data to/from a classic bus card might be limited by the bus bandwidth capacity.
A 6500's capacity and performance can vary much depending on how it's actually populated.
