Well, it depends on which G5 it is. I'm assuming since you're talking about DDR2 you know it's either the 2GHz DC, 2.3Ghz DC or the 2.5GHz Quad (all released in October 2005 and were the last models of G5 ever released I believe). But if it's a model released before them then it takes DDR, not DDR2.
Anyway, these Macs should support RAM speeds higher than 533MHz provided the modules are the right specification, but they will not run at anything higher than 533Mhz because that's all the memory bus supports so as long as you're not bothered about it being underclocked to run at 533MHz then it's OK.
As for the RAM spec, up to 16GB of 1.8v DDR2 533MHz unbuffered DIMMs either ECC or non-ECC can be used. DIMMs can either be 256 MB (non-ECC only), 512 MB, 1 GB, or 2 GB modules that have CAS read latencies of 4 and 5. DIMMs must be installed in the bank slots in pairs of the same size and type; ECC and non-ECC cannot be paired in a single bank
That's from the official specs I believe, so as long as the 800MHz RAM folloows those guidelines it should work no problem but as I say, it will only run at 533MHz.