The mobo I'm considering purchasing, ASUS P6X58D Premium, includes 2 x SATA 6.0 Gb/s ports.
In a 4-drive CS5 system (no RAID) primarily used for photo editing,
1 x WD 450 GB VelociRaptor (WD4500HLHX, SATA 6 Gb/s interface) for OS and applications
3 x WD Caviar Black 1 TB (WD1002FAEX, SATA 6 Gb/s interface) for scratch, docs, archive, and page file
which two drives should I connect to the SATA 6.0 Gb/s ports for best advantage? Or, will it matter at all?
I doubt it will matter much as the prime advantage of the 6 GB/s interface will be seen on large RAID arrays. Why don't you build it and test it with PPBM to see if you find any advantages?
Incidently, unless you find otherwise the Newegg price (today at least) between the 450 WD VR ($270 plus shipping) and the 600 WD VR ($280 and free shipping) is a no-brainer decision for the 600 GB.