There are many aspects to performance tuning ETL environments (CPU (32bit vs 64bit), RAM, disk configuration, network(interface, speed/loading, hops, etc), DBMS server capacity, data volumes, transform complexity, etc).
That is why many organizations need to bring in outside experts to help get them started.
Some general recommendations:
1. More RAM is usually a good thing. ETL tools like to cache data to reduce I/O. This also reduces paging.
2. ETL and RDBMS should be on separate boxes. These boxes should be on the same network segment, the higher the bandwith the better (consider 100 Meg Ethernet a minimum configuration), and be sure to use a switch (not a hub). I would also dedicate a separate network adapter to ETL on the RDBMS box.
3. RDBMS disk configuration for ETL / Data Warehousing.
I could write a book on this. There is way too much to cover in an email and this is very application/environment dependant.
4. Transform complexity / SQL optimization. More material for a book.
In general, you need to look for bottlenecks, try various approaches to mitigate, and measure the impact step by step.
I hope this helps.
Charles Walker