You could look at VistaDB if you are writing in .NET. It is 100% managed code, contains true referential integrity, tsql stored procs, clr procs, and much more in a single assembly you can xcopy deploy.
VistaDB runs in shared hosting asp.net sites under medium trust, and in active directory domains as guest (no local permissions) as well.
There are no registry or other configuration settings required on the machine you deploy the engine on, and the runtime is royalty free.
32/64 bit support is included in the single assembly. Mixed mode engines with unmanaged code generally require you to ship more than 1 version of the unmanaged code to support 32 and 64 bit, or to recompile for specific CPU targets.
See the Advantages of VistaDB SO thread for more information.
http://www.vistadb.net