The sticks don't have to be the same size--I've got the original 2x 256 from Apple and another 2x 512 from memorytogo.
I've never understood why so many people get their panties in a bunch over having to have Crucial RAM. I got the cheapest deal possible by looking on ramseeker.com and everything worked out great. barefeats.com suggests getting the cheapest RAM possible that's G5 spec'd, and even says (in so many words) that expensive RAM is a total waste of money.
http://barefeats.com/memory.html
Also, this should give you an idea of the RAM performance breakdown for the G5:
http://www.xlr8yourmac.com/G5/8GB_RAM_Tests/G5_8GB_ram_tests.html#storytop
In general, at least from the barefeats.com FCP render test (
http://barefeats.com/fcp4.html) video editing performance depends more on processor speed than RAM. I rendered a clip (can't remember how big it was) in FCE before and after I installed an additional gig of RAM, and there was really no difference (I timed it). With video editing, the real benefit with more RAM (at least in my experience) was being able to have my support apps (iphoto/safari/soundtrack, etc.) open without really sacrificing performance - that's where I noticed the before/after difference in responsiveness.