RAID can only be used on seperate drives, unless you have an Intel controller that suports matrix raid. I don't recommend a matrix raid configuration on one drive. I can't imagine the performance being that good since the read/write head will have to jump back and forth between partitions. Also, a single drive failure would take out the whole array, which goes against what raid was designed for in the first place, redundancy.
With raid, the disks are always the size of the smallest disk, in this case 500gb. you'd lose 150gb on the 650gb partition. Also, the point is moot if your controller doesn't support matrix raid.
Intel Matrix Storage Technology can be found on select platforms based on the Intel® 4 Series chipsets, Intel® 3 Series chipsets, 965, 975X, 955X, 945G ,945P, 945PM and 945GM Express chipsets. Contact the platform manufacturers directly to verify which of their products offer Intel Matrix Storage Technology.