We are currently moving from 3 to 4.
Seems kinda silly to go to 3 at this point.
2.6 kernel is a huge win for disk io and memory issues.
Lot of other really big benefits by going to 4 as well, in security (SELinux), various isolation schemes, etc.
Vendor drivers seem to have problems on 64 bit gear on 3 in 32 bit mode.
But 64 bit gear running 64 bit RH3 is a bit lagging, since the 64 bit RH 3 is an after thought (2.4 kernel), as opposed to 64 bit RH4 (2.6 kernel).
I'd think it over.