Get Pro, Home only supports 1 CPU.
Since when do imacs come with more than 1 cpu? Unless you mean cores, Home supports multiple cores just fine.
Theres no reason at all to get Pro unless you are doing some really advanced networking stuff or have a mac pro. I dont know why so many people think Pro is so great, its not, its exactly the same as Home for the average to enthusiast user. Probably 90% of the people who have Pro dont use the features that make it different than Home, theyre just spending a lot more money for a different boot screen.
Regardless I think its best to get Vista 64. Cheaper than Pro, all the same Pro features youll never use in your entire life, and supports dx10. Vista 64 performs the same in games as XP now, possibly better with 4gbs of ram (I say possibly because there arent any benchmarks with 4gbs to prove it), the only reason not to get Vista is if you are low on disk space since it uses about twice as much as XP, or if you are going to run it only in VMware.