According to Cisco, it's recommended to have at least 512 MB of RAM, therefore 4 GB of RAM on AR series should be enough.
You need to consider the type of SRU, for example SRU80 supports full routing table, SRU 200 double full routing table.
The CPU frequency will determine the throughput performance. So it's best to check the number of cores and frequency.
https://www.cisco.com/c/en/us/su ... l-bgp/12512-41.html