Will a 2821 be enough for a gigabit link with projected load of 100Mbps?
I've attached a router performance chart that's commonly used on these forums. In the bandwidth column for a 2821 you'll see it rated at 87 Mbps. This for 64 bytes sizes packets; worst case. Throughput often increases with increased packet sizes, and "normal" packet sizes usually average bigger, but other router services will reduce throughput. Also, you don't really want to run a router at 100% CPU, should have a fair size CPU cushion (about at least a third).
The 2851 might be sufficient, with its rating of 112 Mbps, if the 100 Mbps is the total aggregate bandwidth usage. If you're thinking 100 Mbps duplex, i.e. 200 Mbps aggregate, then you likely will want to move up to the 38xx series.
If the router isn't needed for WAN features, but only for pure LAN routing, you might consider using L3 switches such as the 3560 or 3750 series.