I bought PS 7 and had no trouble...loved it... and the sites I could create with it (excluding those done in Dreamweaver) were some of my best.
After my reformat I lost PS 7 and unfortunately no idea where the disk was, so obtained a (perfectly legal and not downloaded in any way) version of Photoshop CS, which had gone that little bit further than 7.
CS will have a few more bits than 7, which has a few more than 6 etc etc, you get the jist.
At the end of the day you can do sites on all of them to pretty much the same standard, but given the choice i'd have 7 over 6 any day.
Enjoy!