How much memory does your 5510 have ("show ver | i MB")?
Older ones need a memory upgrade to go to 8.3+. If you're not very conversant with the ASA, the upgrade to 8.3+ can be challenging as that is where some significant changes were made in NAT and access-list syntax.
Otherwise the last 8.2 release (8.2(5) from February 2013) is pretty stable and fine for most purposes.
