first point, its not a datawarehousing question, but a general SQL Server
question & Web Developper question.

Second, does your web app is developped in ASP.Net 2? do you cache some
results in you web application?
How many transactions are made in your database? the number of page per
second is not relevant to fine tune your database.
do you really need to support 144RPS anytime? what is your target?

to fine tune...
look at your SQL Server performance counters.
Do you have a memory issue? processor issue? disk bottleneck?
you have to find the problem before trying to fix it.

For your disks:
Look at the avg disk queue length perf counter, if this value is always
higher then 2 (if you have 1 disk only) then the disk is the problem.
use raid 10 or 1+0 disks for the database, put your tempdb database on a
dedicated disk, put your database indexes on another disk. Also the log
files should be on another dedicated disk.

The database himself:
update the statistics of your tables, bad statistics result in bad query
plan and poor performance. (defragmenting indexes is recommanded but more
time consumming)
if your system is too intensive like during a stress test where your start
from 0 rows to 2 millions, SQL Server has no time to update the statistics
of the tables.
For example, the query plan should identify the usage of a full table scan
to find the result because he think there is only a couple of rows, but
there is 2 millions of rows and a full table scan is long, and a index
cluster scan should be better.

if you use storedprocedures, try to add the "with recompile" option which
mean that SQL Server will reevaluate the SP and the query plan behind it.

Start by playing with the database before going at the hardware level.
there is a lot of other tips.
Look at this web site for information:
http://www.sql-server-performance.com/

good luck

Jerome.