Well it doesn't hurt to keep the raptor except in power/heat/space since you already own / use it.
If you're going to buy another 640 anyway, it isn't like you have to change your existing setup other than
to add the new drive and keep your existing drive letters and content as is.
On the other hand, yeah, the raptor is pretty obsolete, and I doubt you'd miss it if you did just partition off part of
the 2nd 640 for an OS partition. That'll be plenty fast enough. The only real problem is doing an image
backup of the raptor on to the new 640 so your software doesn't need to be reinstalled at all;
I'm pretty sure acronis or some such thing can do that image backup and restore to your new/different hard
disk and partition for you.
Now if you wanted speed and reliability, IMHO, I'd buy TWO more 640 drives and put them in a RAID-5
so you'll have 1.2TB usable space and a good redundant RAID to protect your data in case one of the drives
fails. In that case, though, I'd keep the Raptor as an OS disc because you really don't want to be booting from
or installing an OS onto a RAID disc with 1TB of your valuable data on it. Only downside here is that
a software glitch or malware could still wipe out ALL your data.
Alternatively get two or three more 640's and put one or two in an external eSATA or USB enclosure for
backup and leave it powered off when you're not using it, and run the two 640s with or without the raptor
in the case for the OS + data.
Either way, the simplest thing is to leave the raptor in, but if you have the patience to replace it, I doubt you'd
regret the performance difference.