Hi Tzachi Slav,
Yes, You can replace a 3660 router with a 2821 and it can work it out for you. 3660 has a throughput of 51.2- 61.4 Mbps with 100-120,000 PPS switching. 2821 has 87 Mbps throughput with 170,000 PPS switching capacity.
2821's are the new ISR routers with more processing power and memory size. Most of the modules will be compatible between 2821 and 3600's.
The only thing that you will loose out is the Network Module capacity on 2821. 3660 has 6 NM slots while 2821 has only 1 NM slot. So if you have a greater than 1 NM slot requirement, then 2821 will lack that.
HTH, Please rate if it does.
-amit singh