HDD: 3x Samsung Spinpoint F3 500gb in RAID 0. Wouldnt it feel like an SSD? Would i notice a difference?
It won't be as fast as an SSD. Traditional hard drives just cannot compete with SSDs on access time. Even Velociraptors have an average seek time of 4+ ms (Spinpoint F3s are about 8 or 9 ms). SSDs have a seek time of under a tenth of a millisecond. RAID isn't going to make that huge a difference.
Please be aware that you are at least tripling your risk of data loss/failure by putting 3 hard drives in RAID 0. You're welcome to go ahead and do it, but I do hope that you've got a backup strategy and hopefully some sort of imaging software to quickly recover from a failure.