First you might want to validate where 10 gig would be of benefit. Generally it's used where a single gig or multiple channeled gig between network devices doesn't provide enough bandwidth. It can also be used to provide 10 gig to a host.
Depending on your existing hardware, some chassis systems might simply allow addition of a 10 gig card. Others might require additional upgrades to the device before the card can be add. Even more others might require wholesale replacement to support 10 gig.
You also need to consider whether reduced port density might be an issue, or and new issues of device and/or card oversubscription. (For example, 6500 with sup720s provide 40 gig to the slot, but 10 gig cards can be had in 4 port, 8 port and 16 port versions. The latter two oversubscribe the bandwidth to the slot. [There's also PPS performance to consider.])
Also when working with 10 gig, don't neglect the cost of the connectors, and they support the cable and distance needed. (You also need to consider whether the cable will work for 10 gig especially for the distance.)
If you get into much device replacement, you might carefully consider, where possible, using higher port density devices, to reduce some device interconnections, better leverage investment in 10 gig link, and provide possibly higher performance.
(For example, if you have a L2 access edge, and devices host multiple VLANs, and traffic does transit between the VLANs subnets, moving to L3 on the edge should decrease the traffic needed to transit the uplinks.)
