It's too bad you can't just ADD memory to your hard drive like you can your Ram.
You can by using additional hard drives and extending the volume across the additional drive. You need to backup your data first before you do this, however, because the data has to be written to the new sectors.
In the end you are running a software, as opposed to hardware, managed RAID 0, which means it's not as fast as a hardware-controller connected RAID 0, but it is faster than a single drive. The problem though is RAID0 setups are not exactly safe for long term use without a good backup due to lack of fault tolerance. This means if one drive in the pair crashes completely, you will lose everything so having daily backups is more than just a good thing. In this case, they become mandatory!
The way around this issue is to use a RAID 1 or RAID 5. A RAID 1 adds in a third drive, but this is not used and instead works as a parity drive so if one should crash, the other lost bits can be figured out (don't ask me how it's done), and you'll have your data.
A RAID 5 goes one step further with an extra drive or drives setup as a mirror plus the parity of RAID 1. This setup is a bit slower, even a bit slower than RAID1, because data has to be mirrored to one drive while it's being written to the data drive, as well as, to the parity drive. In the end it's quite safe for your data because if a drive should die, the mirror will kick in and fill in the space, and the parity will ensure the data is never lost.
Anyway I don't think you need all that extra info but in the end you can actually extend volumes with Windows NT and up. In Windows 7 this is controlled through the Disk Manager, and in Windows 8 and up it's handled through either Disk Manager, or a new thing which is actually a bit easier to use called Storage Spaces.
John