I am not sure which way to go with a upgrade, I will tell you what I have been told the different s are, please put me straight if wrong.
1. 64 bit let,s you use more than 3 gigs of ram.
2. CS4 is set to run 64 bit.
3 64 bit is still lacking in drivers and compatibility problems with older stuff.
4. 32 bit with 3 gig of ram is easily enough to run CS4 and Lightroom.