6500 cards can only obtain up to 40 Gbps bandwidth, so any card supporting more than 40 Gbps port density oversubscribes the card slot's bandwidth. Using less ports, such that 40 Gbps isn't exceeded, might provide full bandwidth to those ports but you should check how the bandwidth is allocated to ports (since the 40 Gbps per card slot is dual 20 Gbps channels).
Within the 6500 10 gig cards, I recall using every other port of the 6708 or every fourth port of the 6716 will work well with the slot's 40 Gbps.
However, although current literature has the 6704 as not being oversubscribed, before the 6702 was discontinued, only it was documented as providing 100% port wire speed bandwidth. The 6704 was then documented as being very slightly oversubscribed. If it was/is, same might be true with 6708 or 6716 even if only using 4 ports.
PS:
When working with 10 gig cards, you might also want to insure card has DFC (optional on 6704).
