IPSec VPNs between a 515 and 520 are do-able. If at all humanly possible you should try and run the same version of PIX OS on the 515 and 520. I'd suggest either 5.1 or 5.2. Make sure that you have adequate flash and DRAM before attempting to upgrade. Also check your license and make sure that you have VPN capabilities "on" in both PIXen.
Using different PIX OS, such as 5.1 and 5.2 may work, but it will be a little harder to configure and maintain.
For a good intro document see:
http://www.cisco.com/warp/customer/110/pixpixvpn.html
Brian Ford | brford@cisco.com | brford@yahoo.com | 51 75 61 6c 69 74 79 20 6d 65 61 6e 73 20 64 6f 69 6e 67 20 69 74 20 72 69 67 68 74 20 77 68 65 6e 20 6e 6f 20 6f 6e 65 20 69 73 20 6c 6f 6f 6b 69 6e 67 2e | Email me when you figure this out.
