40 gig hard drives are getting diffcicult to find because of the disproportianately high cost for the drives and the usually antiquated nature, not only are large drives often cheaper but they are much faster.
PATA is stupid, its a new name some muppet though it would be a good way to try and switch the names from ATA and IDE to PATA to help distinguish between SATA and ATA, in the end causes more confusion. Looks like your system is IDE although it might have SATA and its not installed youll need to look on the mobo or just check the specs.
You will need to reinstall windows on the new drive as well remember and you might have trouble accessing files on the other drive if its still got its own windows installation.
What i do is partition my hard drive it splits it up so i have one drive but it looks and behaves like its got two a 60Gb for windows and a 120Gb for data which is protected if i need to reinstall windows on that drive. You might want to try this.