This topic needs a huge "It Depends" for the answer. While the answers posted so far are good and correct for given conditions, there is so much more.
Yes, moving off of RAID-5 to RAID-10 is a huge check-mark in the "Do this now" column, but what about the pieces we have not been told?
Thinking about this, I come up with the following items:
Disk layout:
What files are on what drives?
Where is the TempDb and how many files are associated with TempDb
Are the DB's and T-Logs on different drives?
Query management and optimization:
Have the heaviest queries been identified and optimized? How?
Are the Updates causing row relocations or are they able to "Update in-place"?
Row relocations during an update cause all sorts of performance issues.
Index management: Are the needed indexes in place and are they correctly defined
in order to be utilized by the queries which need them?
Index rebuilds:
Indexes become fragmented over time and need to be reorganized or rebuilt. Not
doing this will affect performance.
Environment:
Drive fragmentation can lead to file fragmentation. Even when the drive is kept
clean, a large number of extents for any DB or T-Log file has a negative effect.
Upgrading from SQL 2000 to SQL 2005 or (better yet) 2008 or 2008 R2
is a great suggestion.
Running on Windows Server 2008 x64 or Windows Server 2008 R2 x64 complements the
6GB of memory in natively.
SQL Server 2008 and 2008 R2 have built-in tools for helping to identify problem
queries. In a stressed environment with a stressed DBA, this is worth the upgrade.
Drive Controller Cache Memory:
Is it sufficient? Is the cache shared between multiple logical drives?
Resource competition (pressure):
Besides IIS, what other processes on the SQL Server system are competing for the
system's resources?
Adding a SAN is not a bad idea, but you need to understand the environment. Knowing if the cause of your performance speed-bumps is due to table-scans, improper foreign-key management, row-relocation during update, some other issue, or "all of the above" is very important. Without knowing the underlying cause(s) means the addition of a SAN will only be a temporary boon. Also, knowing your environment is mandatory for the proper sizing of a SAN -- It is not only about the availabe dise-space.
I do not know your environment and could not give a reasonable answer as to what you should do; also my above list is really the tip of a topic to which many thoughtful books have been dedicated.