how much TCP packets should come at 100/1000mbit network card (not an embedded Realtek's home solution) before the Linux kernel will become unable to process the NIC's buffer resulting in packet drop?
Hardware: let's say about dual-core Xeon + Intel's gigabit PCIE card. The final product is planned to capture and process the packets coming from a large net (doing the same job that Netflow does, but with few specific features). But currently, have no test environment available to pre-calculate possible load per server.
Software for such a basic load test: linux's kernel ~2.6.30, netfilter, 2-3 iptables rules. Some kind of network similation is in progress, too.
If all processing is in two iptables rules, we can expect performance 100 MBps and 0.3~0.4 MPps.
Intel PCI-E NIC good choice for this purpose, they are well performance tuning. Dual-core Xeon not the best choice. Core 2 Duo/Qaud >= 3 Ghz or Core i3/i5/i6 faster.