first post - do you have a raid controller? either pci or onboard? then yes you can. raid 0 will give you double the lowest amount just so you know. if you add a 40 gig to that 80 gig and raid 0 is you will only get 80 gigs still. give you put a 200 gig drive on that 80 gig you will still only get 160 gigs instead of 280. and they are just raid drives, they arent specfic to raid 0,1,5.
second post - as far as overclocking, that motherboard is so old chances are it doesnt support overclocking. and even if does those p4 chips run so hot on stock you need some good cooling to overclock it. so an upgrade is your best bet.