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If you're dropping packets due to insufficient CPU, a "faster" router would be needed, this assuming that your exiting configuration is already optimal.
If you're dropping packets due to insufficient bandwidth, you might need large capacity links, and larger capacity links often increase the load on the router, so you might also need, again, a "faster" router.
In the last paragraph I used "might" because even a link running at capacity and/or dropping packets doesn't always "need" more bandwidth, even to support VoIP across it. Any time you have a network link that can be oversubscribed, drops might occur.
Insufficient details to truly say what you might really need.
PS:
The 2921 rated performance is nearly the same as a 3845. Although that's quite a jump from a 2821, w/o details cannot say whether this is "not enough", "just right", or "too much" for your needs.
