do you need two of the same drive to run raid0 becouse i have two 160gig drives but one is a little faster than the other and one is a hatichi and the other is a samsung
I don't think so. Raid0 is striping, the only raid level you would need to worry about size/speed is Raid 1.
Speed doesn't really matter. Size is the most important factor in a raid array.
Bottom line: go ahead. It will work.
So if I wanted to do a Raid1 in the future. Would I need to get another WD Blk 640gb or can I get the 500gb hdd that has the same speed,cache, but different size.
you can raid with different size drives and different speeds, etc with most integrated raid controllers. Its just not considered optimal. in the case of the 640 and 500 you would not be able to use the 140 extra gigs on the 640 in any raid configuration except the JBOD (just a bunch of disks) which is not really a raid level from most points of view. Personally, if you are not too sure about raid then raid 0 is not for you. Its speed advantage in a home computer is questionable these days and it risks your data more than a single drive. But if you want to play, then play.
It's like going for a jog with a friend. It's not necessary for both of you to have exactly the same maximum speed to jog together, but whichever one of you can run faster won't be working to his full potential and you won't get to your destination as quickly as if you were both that fast. But you'll still get there faster than walking.
I fear the Hitachi is quite a bit faster than the Samsung: compare them with Atto on small accesses.
Then, the Hitachi will really be slowed down. A Raid-0 is already slower than a single disk at 80GB per platter, and this is worsening. With two different speeds, the result should be disappointing.