The first step is to image what's on your RAID0 to an external drive. The second step is to test the heck out of that image to make sure that it is good. A bad backup would be a disaster.
Your new primary drive does not have to be as big as the RAID array, only big enough to hold however much data / OS is on it, and a reasonable allowance for working files. How much space is used on the RAID array? As a side question, how big is the RAID0 volume? I would guess that it's only 400 GB; RAID0 can only use the size of the smallest drive on each drive, to the best of my knowledge.
So. Now you have a guaranteed good image of the contents of your RAID0 volume, and you know how much data was on that volume. If it's less than 300 MB, you could even use your 320GB drive as your primary; if it's more, you need a new drive.
Install the designated primary drive and only that drive in the PC, boot to BIOS, and make sure that it no longer thinks it has a RAID array. Make sure that it expects to see a single drive, and that the drive can be seen in BIOS.
Use the image software of your choice (with which you created the image backup) to write the image to your new primary drive, which should be the only drive in the laptop (you'll need a bootable image program, natch). If this is your old 320 GB drive, this will wipe everything on it, so you will be totally dependent on the image.
At this point, I would bet that your PC will _not_ boot. It will get to the section on loading drivers, load a RAID driver, and not be able to read your system disk. If this is the case, boot the PC from the installation disk for your OS and choose a repair installation. This will twiddle the boot sequence and drivers and make you happy again.
NOTE: If you are running Win7, there is probably a simpler procedure to tell the OS "The next time I start you up, there will be no RAID," but I don't know it. Community, can you help out with this? Thanks.