SQL 2005 Server Performance And Maximum Memory Pro

A query was taking 20 seconds and consuming 70% CPU takes only 1 second after setting Maximum Memory property to 2048 MB - why?

Server:
OS Microsoft(R) Windows(R) Server 2003, Enterprise Edition
Version5.2.3790 Service Pack 1 Build 3790
8 GB memory
Two Dual-core AMD Opteron 285 2.6GHz Processors
Server is not configured for AWE
Fiber channel connection to EMC Clarion - two LUNs - one for MDF, one for LDF

SQL 2005
SQL 2005 32 bit Standard Edition - SP1 (version 9.0.2047)
Three instances installed on server - only one instance in use
Binaries and system databases on local mirrored disk
Database file (MDF) on one EMC LUN - dedicated physical drives
Log file (LDF) on one EMC LUN - dedicated physical drives

Query in question:

SELECT TOP 10 Address.Address1, Address.Address2, Address.City, Address.County, Address.State, Address.ZIPCode, Address.Country, Client.Name,
Quote.Deleted, Client.PrimaryContact, Client.DBA, Client.Type, Quote.Status, Quote.LOB, Client.ClientID, Quote.QuoteID, Quote.PolicyNumber,
Quote.EffectiveDate, Quote.ExpirationDate, Quote.Description, Quote.Description2, Quote.DateModified, Quote.DateAccessed, Quote.CurrentPremium,
Quote.TransactionDate, Quote.CreationDate, Quote.Producer FROM ((Client INNER JOIN Address ON Client.ClientID = Address.ClientID) INNER JOIN Quote ON
Client.ClientID = Quote.ClientID) WHERE (Quote.Deleted = 0) AND ((Address.AddressType)='Mailing') ORDER BY Client.Name


Address table - 161,075 rows
Client table - 161,634 rows
Quote table - 59,145 rows


With default maximum memory setting (2,147,483,647 MB) - query runs in 20 seconds and consumes over 70 % of the CPU.

After changing maximum memory setting to 2048 MB, query runs in less than 1 second.


Question is:
What is the best practice for setting the minimum and maximum memory settings for SQL 2005?
What can be monitored to identify the cause of these type of issues - using profiler, PerfMon, other tool?

Thanks