Which 64 bit compiler for a DLL called from ASP.NET?

My next online app would be best written in 64 bit. I plan to make a 64 bit DLL of the functionality, then interface it with C#.

Which compiler best supports this? I've got VS2010, but it doesn't automatically or easily compile 64 bits, so I was wondering if buying a new compiler would speed things up for me...? VS2013?

And are there any gotchas with 32 bit vs 64 bit and ASP.NET?