With such a huge push in the industry to consolidate "tin" (aka servers), you have a number of options available to you, when consolidating SQL Server instances. You can go single instance or muti instance, orthogonal to this is whether 32bit or 64bit suits your needs. 

I hear you ask "So which is the best approach ?", it depends on what your are doing in your code, how many DB's are involved, how big/varied is each DB, how big/varied is each workload and what are your business requirements ?

Mike Ruthruff has written a great Whitepaper that outlines some of the factors that you will need to consider. As with all things in life, it is important to weigh up the pro and cons, and IMHO Mike has done a great job in clearly illustrating that it really depends on what your application does, but that 64bit is something that you should really consider.

You can get his Whitepaper here: SQL Server Consolidation on the 64-Bit Platform — Lessons Learned