You can have 2 unindentical drives setup in an array but the system will only consider both drives to be of similar size as the smallest one.
Also XP does not support mirroring, only simple, spanned and striped. None of which are as secure or as fault tolerant as mirrored.
You can buy hardware and software capable raid array but you'll be paying high for it! You could always get Server 2003 or similar, this is capable of all RAID categories. 0,1,2,3,4,5 and 10 etc.