There are some very compelling reasons to upgrade to SQL Server 2008. I'll cover a few here but haven't got a huge amount of time so apologies for appearing curt:
Improved ETL performance with Persistent Lookups and Increased Parallelism
Merge statement and change data capture which is highly valuable if you've ever tried to bulid a warehouse
Please see my blogs at: http://community.altiusconsulting.com/blogs/altiustechblog/archive/2008/09/03/sql-server-2008-part-1-ssis-pipeline-scalability.aspx
On the SSAS side it now does subspace computation automatically i.e. doesn't calculate over sparse cells which could make a significant difference to calculation bottlenecks. The tool improvements in SSAS will help developers to build better performing cubes.
The most compelling reason to upgrade your relational engine is that it now does compression on the IO. Intuitively you might think that this will slow it down. However in some cases it caused an increase in performance since with 64 bit servers packed full of memory it is the IO that is now the bottle neck. Freeing up this bottle neck by using compression can imrpove performance.
If you're buying SQL 2005 then I'd definitely skip to 2008 since it has a legacy clause meaning that you can use 2005 (I think there are some conditions). So buy 2008 use the bits from 2005 you want and upgrade when you're ready. They'll run side by side.
There are many more improvements that I'm hoping to detail in my blogs.