It depends largely on the amount of data you are going to handle as well as the complexity of your script and expressions, number of objects per sheet, number of concurrent users, number of documents...
This is what the documentation says for version 9 SR6
3.1 System RequirementsIn order to successfully install and run the QlikView Server/Publisher, the followingbasic requirements must be met by the system:Hardware and Software• 1 GHz (x86 processor) or 1.4 GHz (x64 processor). 2 GHz or faster, withseveral cores/processors recommended.• QlikView Server will use the color settings of the Windows server where itruns when sending charts and other graphics to the client. For best results,the color palette on the Windows server should be set to at least 65,536 colors(16 bit).• a mouse or an equivalent pointing device supported by Microsoft Windows.• (optional) a DVD drive for DVD-based install media only.• a hard disk with at least 450 MB of free disk space.• 1 GB RAM minimum on x86 systems and 4GB minimum on X64 systems..The server's capacity to publish QlikView documents and the number ofusers who concurrently can connect to it are strongly related to the amountof RAM available.• An http server for providing QlikView Java, Java Objects Client and AJAXZFC solutions to end users (e.g. MS Internet Information Services (IIS) orthe built-in QVWebServer). Microsoft IIS or the built-in web server isrequired when using tunneling, external authentication or NT security withJava clients and whenever the AJAX Zero-Footprint client is used.• TCP/IP Network.• Microsoft .NET 3.5.Actual requirements will vary, based on system configurations. It is recommendedthat you work with your local QlikView representative to configurean appropriate hardware platform for your QlikView Server/Publisherrequirements.• Microsoft Internet Explorer 7 or Firefox 3 to use QlikView ManagementConsole.24Supported Operating Systems• Microsoft® Windows Server 2003 ™ including x64 Edition• Microsoft® Windows Server 2008 ™ including x64 Edition and Server 2008R2*• Microsoft® Windows XP™ including x64 Edition**• Microsoft® Windows Vista™ including x64 Edition**• Microsoft® Windows 7 including x64 Edition***To ensure full functionality of the QlikView Server, we recommend that the User AccountControl is turned off in Windows Server 2008.**Recommended for development and testing purposes only.Database requirements and recommendations for QlikViewPublisherThe database in QlikView Publisher can be either a Microsoft SQL Server oran XML repository that requires no preinstalled software.The supported versions of Microsoft SQL Server are SQL Server 2000, SQL2005 or SQL 2008. If you have a Microsoft SQL Server already set up werecommend using that. The XML repository is sufficient for most installationswhen it comes to performance.If you do not have a Microsoft SQL Server available we recommend thatyou start with an XML repository installation and upgrade to Microsoft SQLServer if the performance is insufficient. It is possible to migrate all data inthe database between XML repository and SQL Server.