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:
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?
Indexes become fragmented over time and need to be reorganized or rebuilt. Not
doing this will affect performance.
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
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.