I have 2 raptors in raid 0 and was wondering if it is possible to change my stripe size without reformatting?

AFAIK, you cannot change the stripe size without losing all of your data. I suggest you make an image of your current setup on a spare drive, change the stripe size, then ghost the image back onto the new RAID setup. I'm not sure if you would even notice a difference in performance going to a different stripe size, but at least my suggestion gives you an option. What size do you currently have and what size do you want?

i have a 16k stripe size and i want to test diferent ones and see whats the best

I have no firsthand experience, but differences will not vary greatly. From what I recall, the smaller the stripe size, the quicke small data files can be read (i.e. Word Docs). The larger the stripe size, the quicker larger files can be read (i.e. game data).

However, I may have that backwards.

Nonetheless, the general purpose is 64K blocks.

If you are gonna test this for your own experience, I would suggest using the smallest block size (I think it's 8K), then movingto 64K, then 128K (if that's the biggest....).

Otherwise, you MUST reinstall everything on your HDD's after changing the block sizes.

pritchett beat me to it, but as pckid777 said, it shouldn't matter that much either way (only noticable in benchmarks most of the time). is yours @ 64k? why do you need to change it?