If it were mine, I would not do it. Unless I were to put the two smaller drive in RAID1, which would give a total capacity of 250 GB but have two copies in case one drive failed.
Best place to start with RAID if you want to tinker with it is using the motherboard's chipset. You can then build a RAID array in BIOS. However, you will have to change the drive controller mode from the default of IDE to RAID. If you already have a running OS, it won't boot after that, since it will have the wrong drivers.
So if you have bare metal, set the drive mode to RAID, build a RAID1 from the two smaller drives, and install your system. Win7 will be easy; WinXP would require a driver floppy during the installation.
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Whether or not to use RAID, and which level of RAID to use, depends strongly on what you want to accomplish. Joining two drives in RAID0 will give double the capacity of the smaller one and double the speed of the slower one. But, it will be more likely to fail than a single drive, and more difficult to recover should it fail. Do frequent backups.
RAID1 just keeps two identical copies of every sector, one on each drive. If one drive fails, it runs happily off the other and, when you replace the failed drive, will copy the surviving drive's data to the new drive and be back in RAID1 mode.
Other levels are compromises, with some disks containing data spread-out a la RAID0 and one or more disk containing parity information that can be used to recover the contents of one (or even two) failed drives.
In my mind, there are only two reasons to use RAID. Either you need one of the features (greater speed or capacity by combining drives, or surviving the failure of one drive) or you want to muck around with it as a learning process. If you can't figure out a specific benefit that you will get from a specific RAID level, don't use it.
Read Wikipedia on RAID levels or talk to a knowledgeable friend.
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As to "mobo vs. Windows", the process of combining the various disks into one volume can be done at the Windows level (slow and eats your CPU, but simple), by your chipset (faster for RAID0 and 1), or with a separate and potentially expensive RAID controller (well worth it for high-performance production systems). As with RAID levels, there are advantages and disadvantages to each choice, and the correct choice depends on what you need to accomplish and how much money you have to invest. I personally own a datacenter-grade external RAID array that takes eight drives, but I use it to play with and learn from. Got it surplus for only a grand a few years ago.
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