Hello Patrick,
if you want to handle multiple full BGP Internet tables you need a powerful supervisor like SUP720 3CXL.
The number of iBGP peers has less impact because if they share the same outbound policy BGP will put them in the same update-group (even without defining a peer-group).
Verify if a 6504 can support this supervisor otherwise you will need a bigger chassis even if the forwarding requirements can be fullfilled by a single linecard.
Choice the latest series linecard to have better performance (less ports sharing the same resources).
Install on them DFC 3CXL.
Hope to help
Giuseppe