I'm sure it would! But since I can't afford to go with one of the higher speed 8-Cores, I can't help but wonder if a Quad with faster processors might not actually deliver more immediate bang for the buck when it comes to heavy sample library and VI use. Seems there aren't many strong opinions about this, though....
Unless you find a user working regularly with logic on both 4 and 8 core systems, you have to expect that people will report their findings with the one system they work with.
I have the original 2007 Dual Quad Xeon 3.0. Benchmarks I carried out showed it to have around 6 times the CPU power of the 2 x 2.5 G5 I had been using prior to that. The combination of Logic 9 and OSX 10.6.2 seems to have some optimisations with respect to spreading the load over the 8 cores, but I have to say, I never had any problems with logic 8 in 10.5 on this machine.
If you can't afford to stretch to an 8 core, the 4 core will still be a major jump from your G5. If you need that power now, the decision should be easy. If you don't need it, wait until you do before spending the money on a new mac. Remember, no matter when you buy, within a few months there will be some faster machine available. The whole trick is coming up with a definition of "need" that is relevant for you.
Oh, irrespective of the machine you buy, budget for plenty of RAM, 4 GB is an absolute minumum, I would say put in at least 8, particularly regarding your statements about heavy sample use you can't have too much RAM. Within the next months we will see more and more 64 bit software to match up with Logic 9.1's 64 bit capabilities, this is what heavy sample users have been asking for for a long time.
kind regards
Mark