SQL 2005 Cluster 64-Bit & Memory

We are replacing an existing SQL Server 2005 32-bit environment with a new SQL Server 2005 64-bit Enterprise High availability environment. The new solution will be a 2-node Microsoft Cluster with SQL Server Cluster Server, all 64-bit Enterprise additions. What I have a problem detemining is what is a good baseline for the base memory to install. I will assume that the 64-bit OS & 64-bit SQL will require more memory than the Win2k3 32-bit solution. We will have one instance running initially but we anticipate adding more instances later (probably 2 more). The Dell DL6850 servers have 8 GB but I fear we may need more. Can you suggest a minimum amount of memory to handle the 64-bit OS, APP & 2-3 instances?

You are good to go for installations. Analyze your applications memory requirements to add more. Capture these counters when your application is running
1) Memory
2) SQL Server Buffer manager
3) SQL Server memory manager
Check what is total and target memory , available memory for OS, Page faults etc. to get some ides on current/future memory apetite.
Thanks
Saurabh

Welcome to the forum.
I would suggest you to first go with one instance and monitor the performance of your server(You can see below performance counters to gauge server performance). Once you are happy with server performance, you can go ahead and add more instance.
Performance counters
Disk
1 Physical Disk: % Disk Time
2 Physical Disk: Avg. Disk Queue Length
3 Physical Disk: % Free Space
4 Logical Disk: % Free Space
Processor
1 Processor: % Processor Time
2 Process --> % Processor Time
3 System: Context Switches/Sec
4 System: Processor Queue Length
User Connection
1 SQL Server: General Statistics: User Connections
Network Connection
1 Network Interface: Bytes Received/sec
2 Network Interface: Bytes Sent/sec
3 Network Interface: Bytes/sec
4 Network Interface: Output Queue Length
Memory
1 Memoryage/sec
2 Memory:Available Bytes
3 SQL Server: Buffer Manager: Buffer Cache Hit Ratio
4 Physical Disk: Disk Reads/sec
5 Physical Disk: Disk Writes/sec
6 SQL Server: Memory Manager: Total Server Memory(KB)
7 SQL Server: Memory Manager: Target Server Memory(KB)
8 SQL Server: Buffer Manager—Page Life Expectancy