Quick answers :
First question : yes, you will see a notable difference in speed with the Raptor drive, especially if your virtual memory also swaps on that drive (and I imagine it would). I would also do any converting and stuff like that on this drive before writing away the end result to the other HD.

Second suestion : you shouldn't have any problems with putting the OS on a SATA drive. If the SATA interface of your motherboard is directly implemented in your chipset the drive would appear in your BIOS like any other HD. However, if your motherboard uses a third party chipset to implement SATA, you will need to install a third party driver when installing XP (by pressing F6 and poining towards the driver when the installation wizard asks you to)
In any wase, it shouldn't be any problem.
I've only got experience with Intel chipsets though, I have no idea how well this is implemented in an AMD chipset. But I can't imagine it would be a whole lot different.

Faster RAM is always a good thing, but you'll have to check what the maximum speed of RAM that your motherboard supports is. Obviously, if your motherboard only supports PC2100 RAm, you won't see any benefit from using faster RAMs. The other thing you can do to speed up stuff is by running in a RAID0 configuration, but you already stated that you don't want to do that. The last thing I can think of is upgrading your CPU, but I don't think you want to do that because I'm sure you'd have mentioned it otherwise.

Can't help you with the HD capturing issue, but maybe the Plextor ConvertX PX-M402U is a solution for you. This little unit captures video and converts it on the fly in real time to mpeg1, 2, 4 and DivX. It costs about $140 so it's well within your price range.