Good one..

First 2 queries are non-sargable. i would like to write the first 2 queries in a different way.

Find records which date falls somewhere inside the current week.

where TransDate >=
dateadd( week, datediff( week, 0, getdate() ), 0 )

Find records which date occurred last week.

where TransDate >= dateadd( week, datediff( week, 0, getdate() ) - 1, 0 ) and 
Transdate  < dateadd( week, datediff( week, 0, getdate() ), 0 )

I have an article on datetime.

http://beyondrelational.com/blogs/community/archive/2010/09/30/top-10-questions-on-datetime-asked-in-forums.aspx

You can find more such queries here http://beyondrelational.com/blogs/madhivanan/archive/2010/06/21/understanding-datetime-column-part-iii.aspx

I agree with Rami that the first two queries could be horrible on a large table. In most cases this type of queries will result in table scans.

Indeed, don't promote functions on columns in where clauses. Use foldable expressions in stead as RamiReddy D showed.

Agreed. The last two tips demonstrated are really cool. Many times we need to do calculations to find start and end date of a given period (week, month etc) and the logic demonstrated is worth noticing. It goes bad only when applied on a column.