32Bit to 64Bit Server and few Questions ?

Hi there,

1) Our company is buying new 64bit server to upgrade our datacenter.
Actually we have 2x 32bit CentOs servers and i wonder if i install CentOs 64bit and transfer all accounts to the new server if it will have any impact.

2) Also is it better to use 32bit or 64bit for web hosting server and actual software ? Is there any compatibility issue ?

3) What is the best partitioning table for a 2x 1.5 Terabyte web server.
Im planing this

/tmp
/
/swap x2 memory
/backup

Or simple Mirror Raid ?

Can someone guide me on best now a day practice thx

4) What centos version to use ? 4.x or 5.x ?