Although it is always recomended to use the same specification drive when creating a RAID array, using different drives will result in only operating as fast as the slowest drive. Would it be a bad idea?...In your case, Not at all. Since your only using 2 drives in RAID 0, the different cache just doesn't make a difference. Drive cache would be more important perhaps in a very large server/storage envronment with more IO requests.
Areca Technologies - US Channel
***** RAID, no matter how redundant, is not a substitute for proper and regular backups *****