Hmm. Didn't even realize Sata had different versions. If I recall both HDs are a WD Caviar Green. Only old thing from the old build is my DVD writer, powersupply and the hard drives. The rest is all new parts.
The whole SATA versions thing is irrelevant for Hard Drives as even the fastest consumer hard drives don't max out Second Gen SATA speeds (SATA 3 or 3Gbps) New drives are built to the SATA 6 Gbps standard, but they don't actually saturate the connection. SSDs on the other hand have for some time now and even top what SATA 6 can offer now, but for a budget system that's nothing to worry about (No need to spend the money on a PCIe SSD over a SATA one unless you need the best of the best). I would still try to find money for at least a 256GB SSD for the OS and important applications. I've had solid luck out of the ADATA SP550 and the OCZ Trion 150 lines for currently offered budget models. You can argue all day long for more expensive SSDs, but the jump from one of these to something like a Samsung 850 EVO isn't noticeable day to day compared to the higher price, at that point I would spend the extra on a bigger version of the cheaper drive.
As for hard drives, sadly the Caviar Greens are performance wise the crappiest and slowest hard drives of their generations, with that said they tend to be reliable and pretty good as far as noise levels so they'll still pair alright with a new SSD.
Already having the PSU makes it not as silly of an option (though it was major overkill to begin with I'm sure as well). I certainly wouldn't go out and buy another already having it on hand from the old system, just if you were guying new it wouldn't be a very good choice.