Hello,
515E is an enhanced version of the PIX 515. It provides stateful firewall and IP Security (IPSec) virtual private networking (VPN) services. Depending on the Cisco PIX 515E model, VPN functionality is provided as a service of the Cisco PIX OS or through an integrated hardware-based VPN Accelerator Card (VAC) delivering up to 63 Mbps throughput and 2,000 IPsec tunnels.
Visit the following page for more differences:
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_qanda_item09186a0080091b17.shtml
Not sure about the number of tunnels w/o H/w accelerator.