Hi,
You can mix 32 bits and 64 bits in the same farm, it's fully supported.
However, it's highly recommenced to keep each tiers in the same bit width (for example all WFE 64 bits, application server 32 bits, SQL servers 64 bits).
From msdn @ http://technet.microsoft.com/en-us/library/cc261700.aspx
You can deploy an Office SharePoint Server 2007 farm on a mix of 32-bit and 64-bit servers, provided that you maintain architecture
homogeneity at each topology layer. For example, you should not mix 32-bit and 64-bit Web front-end (WFE) servers, but you may use 64-bit database servers in a farm with 32-bit WFE servers and application servers.
Warm Regards, Pratik Vyas | SharePoint Consultant | http://sharepointpratik.blogspot.com/ | Posting is provided "AS IS" with no warranties, and confers no rights