You will end up with a 240gb drive, throwing away a lot of storage - the perfrmance benefits of raid0 are questionable for most normal use, and almost certainyl not better than what you get by having your page file on a separate drive.
My bias is to use my smaller drive for OS and programs and the other for media, storage, and workspace. Having the pagefile on a separate drive is beneficial if you actually use the pagefile; if you have a lot of Ram, it may not have any effect at all.
The biggest gain from multiple drives for me is in encoding; if your system is relatively powerful, having source and final files on separate drives can make a huge difference. (I used to notice this a lot for adding audio and the like to divx files; my system was never fast enough to benefit while actually encoding to divx, because the simultaneous r/w performance of the drive was always enough to keep up wth the system's ability to encode, and I alwasy had enough ram in the system to avoid hitting the pagefile during encoding).
edit - because I'm using a motherboard with decent onboard raid, I've always planned to add a third hdd to have my OS on a raid0 partition, but everytime I go to buy an new hdd, it's much cheaper to just buy a bigger drive, as available capacities increase, and so it's never seemed worth it.