Payment Gateway Requirements
The Payment Gateway function would normally be met by a Web Server/Application Server capable of supporting Java Servlets. Other configurations are certainly acceptable. Mod-perl and fast-CGI enabled Apache servers should have very good performance.
Since most of the work of the Payment Gateways will be done using SSL, the CPU load created by SSL encryption may be significant. This load can be minimized with dedicated SSL-encryption co-processors installed in the server.