Hi Luis,
Go for 8.3 further go through this link for lemitations and pre requsites to upgrading
https://supportforums.cisco.com/docs/DOC-12690 (will give complet guide)
Pre-Requsites to Upgrading
Many models of the ASA require a memory upgrade prior to upgrading the ASA to version 8.3. Brand new ASAs from the factory (manufactured after Feb 2010) come with the upgraded memory. However, if your ASA was manufactured before February 2010, and is one of the models below requiring a memory upgrade, then you will need to purchase the memory upgrade part prior to installing 8.3 on your ASA.
| Platform | License | Pre-8.3 Memory Required | 8.3 Memory Required | Memory Upgrade Part Number |
|---|
| 5505 | Unlimited (inside hosts=Unlimited) | 256 MB | 512 MB | ASA5505-MEM-512= |
| 5505 | Security Plus (failover=enabled) | 256 MB | 512 MB | ASA5505-MEM-512= |
| 5505 | All other licenses | 256 MB | 256 MB | No Memory Upgrade Needed |
| 5510 | All licenses | 256 MB | 1024 MB | ASA5510-MEM-1GB= |
| 5520 | All licenses | 512 MB | 2048 MB *
| ASA5520-MEM-2GB= |
| 5540 | All licenses | 1024 MB | 2048 MB *
| ASA5540-MEM-2GB= |
| 5550 | All licenses | 4096 MB | 4096 MB | No Memory Upgrade Needed |
| 5580 | All licenses | 8-16 Gb | 8-16 Gb | No Memory Upgrade Needed |
