Will exceeding 2000 documents in a SharePoint 2007 Library folder degrade only the performance of that library?

2000 is the view threshold. Index can be added

Can we use XOR in a calculated column in SharePoint 2007

 IF, AND, OR and NOT  are supported. XOR is not supported.

how to find out name of list who have particular custom column

iterate through all sites and list

[void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint")
$spSite = New-Object Microsoft.SharePoint.SPSite("http://url")
$webApp = $spSite.WebApplication
foreach ($site in $webApp.Sites) {
    foreach ($web in $site.AllWebs) {
        foreach ($list in $web.lists) {
            if ($list.fields["Custom Field name"]) {
                "List has custom field" | Out-File C:\filePath\log.txt -append
            }
        }
    }
}

validating only a part of a web Page

This can be implemented by using validation groups.

Enterprise Level Deployment: Features and solutions

Ted Pattison says "packaging and deploying your WSS development efforts using solution packages is a best practice, and knowing how to do this should be considered an essential skill." 

There are numerous ways to package   SharePoint solutions, from manual creation of manifest.xml and diamond directive files (DDFs), to Visual Studio projects that use the Windows SharePoint Services 3.0 Tools: Visual Studio 2005/2008 Extensions (VSeWSS).

LIst of custom log viewer

• SPLogViewer - supports loading and filtering one file, does not follow/tail log displays data in a DataGrid.
• SharePoint Log Viewer  
• Stonebond SharePoint Log Viewer  
• ULS Deobfuscator - 
• SharePoint ULS Log Parser - one click access to log files, supports filtering by Process Name, Level, Area and Category.
• Log Viewer Feature - access from Central Administration, select individual log basic filtering on category, event and trace severity, output as HTML in Central Administration.
• WSS / MOSS Log File Reader - Access to individual log files from Central Administration, standard DataGrid filtering and sorting options.
• Stefan Gordon's ULS Viewer - Open individual log, does not follow/tail log, displays data in ListView detail. Filter by secerity, process category and free form text.
• SQL Integration Services - All reporting, sorting and filtering options provided by MSSQL (Blog Post with ETL Package Download), time consuming to setup.
• LogParser - Familiar interface to IIS Administrators, pleanty of options. Setup takes too long for small installations.

is there a way to view who is requesting access to a site?

Yes you recieve a email for request but there is no form to see it. At most you can configure the email for same Site Actions > Site Settings > Advanced Permissions (under "Users and Permissions" group ) > Settings > Access Requests

Assign permissions to library view

Sometime a solution came to have views with different column for different set of user. No its not possible. 

How to chnage application pool that a web app is using.

No simpler out of box solution. need to use powershell
http://www.toddklindt.com/blog/Lists/Posts/Post.aspx?ID=276

Pointing Your SharePoint Servers at Themselves

 make sure that every server in the SharePoint farm points to itself for all web apps. If  something goes wrong we can  use RDP to log in to each server and try to pull up SharePoint. If this attempt works, then I know that the server is working