Currently have a 150 GB Velociraptor for OS and apps and a WD 250 GB for data (mostly photos and home movies), and both drives are getting a little tight. Backup is on NAS, so that's not a concern.
Was thinking of getting another 150 GB Velociraptor and putting it in RAID 0 for my OS and apps, but I'm wondering if there are better options for the $150 it would cost, such as the 640 GB WD SATA 6.0 GB ($80 on Newegg)?
Judging from the wording of your post, it sounds like your primary concern is space, not performance. If so, rather than paying a premium for the performance of another Velociraptor I'm thinking that you'd likely be better off getting a cheaper drive with more capacity.
Leave the existing Velociraptor as the OS and application drive, because performance is more important for that. Install the additional drive and use it to store documents, music, videos, photos, etc., for which performance isn't as big a deal but which require a lot of space.
Doing that also avoids the increase risk of RAID 0 - if you EITHER drive in a RAID 0 set fails you loose EVERYTHING stored on BOTH drives.
If you're not already doing it, a good way to spend the extra money you save is to buy an external hard drive and use it for backups.
The Velociraptor is down to 50 GB free with Windows XP and apps. Will be upgrading to Win 7 when I do my rebuild, so I don't know if that takes more or less space than XP? At what point should I get concerned? If I need more space I'd rather upgrade it now when I have everything taken apart!
You can't do an "upgrade" from XP to Windows 7, so you're looking at a clean install (you can buy the upgrade media if you're replacing XP with Win 7, but it can't be used to do an "upgrade in place"). Be sure you have all of your install kits and license keys.
I have a 160GB SSD with 64-bit Windows 7, Office Pro 2007, Visio 2007, Visual Studio 2008, Adobe Web Premium CS4 (includes Photoshop, Illustrator, Dreamweaver, Acrobat, Bridge, etc.). I have no page file, but my hibernation file is about 10GB in size. I also have my home folder on the SSD, although I don't store any documents in it. All of that uses up about 55GB on my drive. The bare Windows 7 installation was about 10-12GB, everything beyond that was the applications, restore points, the home folder and the hibernation file.
It depends a lot on what apps and software you use, but my guess is that a 150GB drive will be fine for you unless you've got a lot of bulky games.
For best space efficiency, I'd recommend buying the new drive and connecting it while your old XP system is still running. Copy any files you need to keep over to it (perhaps even clone the whole disk and test that you can boot from it as proof that you haven't burned any bridges), then install Windows 7 and delete/recreate the OS partition. That way you'll have a clean, fresh OS disk with no left-over junk on it.
Thank sminlal! Bad choice of words on my part. Yes, it will be a clean Windows 7 install, not an upgrade. I do have a fair number of games installed - both mine and my 4 year-old daughter's! 300 GB for OS and apps would give me plenty of room to grow, which is why I thought of adding a second Velociraptor in RAID 0, but it sounds like the 7,200 RPM drives with 32 MB cache aren't too far behind. I just hate wasting a perfectly good drive.
An SSD will certainly give you a lot faster boot times and faster load times too, for those applications that are installed on it. I personally wouldn't undersize the SSD too much, though. You have to decide on a reasonable trade-off between cost and the time required to hand-hold your drive so that it doesn't fill up.