- Size of database ?
- Size of log ?
- Clustered indexes are id or composite ?
- Server run only SQL Server ?
- SQL is standard or Enterprise ?

Solutions steps with priority:

- tune the database
Use the Profiler for capture a trace of the critical transactions,
then use this trace like input for the Optimization Index.

- move tempdb into another disk
- move logs into another disk

- Capture Performance SQL server / CPU counters and analyze.
with this data, choice to:

- add a raid 5 for data
- add memory 2GB to 4GB
- add CPUs

Normally multiple CPU is correlate with multiuser access,
not with single query performance.

- Split Database into two servers is the last solutions
( but multi server joins and can slow down very much )

Tune is the most important,
with a good index you can speed up a query of 100000 times;
with hardware you can speed up by 2-100 times ( with a exponential cost )