You might want to start a new thread for this, but I'd say neither one is really going to help a whole lot with speed in Photoshop. Bumping the RAM would probably help most, because it could move some HD processes to RAM--but you're really talking about a small difference in total memory, from 512 to 768, so I wouldn't expect a huge improvement. People see the huge improvements when they double or triple memory, especially when they're starting with a really small number (like going from 128 to 640, or 256 to 1 gig).
HD space becomes more of a factor when your HD is close to full, because that gives Photoshop less room to use as a scratch disk. So if you're pushing 50 GB used on your HD, that might be the thing to upgrade first. Others will have to speak on the 7200 vs 5600 difference--I can't say how much that would help. They'd probably need more info, though (typical file size, filters, etc. that you use).