depends on the website traffic, bandwidth would be the main constrain. also check with the TOS of the server provider. Mostly they allow proxies that are specific to user group rather than open proxies where you browse anything. athlon 2800, 2 gb ram (1 gb should be minimum if you are using cpanel) would be fine as i had used it for one of my proxy (which i already sold). it handled 50k unique users a day. Better you get directadmin panel rather cpanel as it needs much ram.