iMac 8.1 2008 are 64 bit.
See here to find your exact model info.
To boot your iMac in 64 Bit mode, you need to hold down the 6 and 4 keys when starting up.
To restart in 32 bit, just restart normally without holding any keys.
If everything is working well in 32 bit, stick to it.
The only reason I see to boot in 64 bit, is because you would use a software that only runs in 64 bit.
BUT there might be other advantages .