Your original proposal is fine.
Only thing I would do differently is to use your SCSI drives in this fashion:
2 for your Windows Pagefile in RAID 0 (striped) and then the same (the other 2 drives) for your Photoshop scratch disk.
It's only 18GB of storage (16 and a bit I guess in reality) but how much RAM are you using? You mentioned 500GB PSD files - surely you meant 500MB? If you truly meant 500GB then you need a new system and need to tell us what you're doing that's making 500GB files :-)
18GB is plenty for Pagefile and PS Scratch and 15,000 RPM SCSI in RAID 0 will be lovely and fast. XP will automatically create additional pagefile on any available HDD (starting with the one the pagefile currently resides on and then going to the OS) in the event that it runs out. This will bring your system to a screaming halt until it's done it (usually) but should never happen. I have a system with 4GB of RAM and with PS CS 3 open with multiple images including 1 x 250MB "open as smart object" with various layers added, Outlook Open, IDImager Open, WinMedia Player Open, a 3D game open and running, IE 7 open and all the other stuff that's tyically going on in the background (at boot, my system has about 15% of RAM utilised of 3.5GB available) I could still only manage to use just under 3GB of pagefile. For that reason, my pagefile is 4GB in size. If you had any concerns, since you have a dedicated drive for it, make the Pagefile a fixed 12GB in size.
In PS, you can change the file info to show you how much PS is paging out, too, and I'm sure you'd find that the 18GB stripe will be plenty.
Oh, I'd also just use a 160GB for the OS and apps, but if you want to go with 300GB there's absolutely no problem - just more space than you typically need when all you have is the OS and your apps and all data and pagefile etc is elsewhere.
I can't see why your system wouldn't support a SATA controller and SCSI controller - your BIOS will just see the SATA controller as a SCSI controller anyway.
Depending on your price point, case capacity and such, you could even consider getting 2 x 1TB drives for your data and running in RAID 1 (relatively cheap SATA RAID controllers with 2 ports are available from quality manufacturers such as Adaptec that support SATA II) giving you redundancy. This doesn't change the need to backup, but it does mean availability of data constantly even if there's a single drive failure.
The combination of the SCSI pagefile and scratch disks, freeing the pagefile from the OS drive, and bringing your data locally through the bus instead of via the network, will make your PC seem like it's 10x faster than it is now. Also, the SCSI and SATA controllers (and SATA RAID if you go that way) will transplant nicely into a new system down the track if you want to go that way (and most new quality motherboards will have various RAID controllers on them anyway, giving you more options).
Finally, take a look at
http://www.techbargains.com/hottips/hottip12/index.cfm if you want to get a few more tips for improving your system performance. Also the additional links from that page. As all of the pages say, though, making ANY changes could risk crashing your system if you get it wrong (or even if you get it right) - so I disclaim any and all responsibility in the event something happens. The main secret is that if you don't understand what they're suggesting you do - don't do it.
Hope that helps!