software raid performance question

By software RAID, did you do "Striped" mode in Windows Disk Management? Or did you do "Spanned"? A few things can come to mind explaining your poor performance:

Your Seagate is a performance drive with greater areal density and 32MB cache, while your IDE WD drive is 8MB cache, in a RAID0 array, your faster drive would be slowed down by your other drive.

Unequal disk sizes, as RAID0 is a stripe, so that means the striping pattern would only carry as long as the smallest partition, in this case, 250GB. You'll still see a 750GB partition, but the performance won't be as stellar as say, two WD 6400AAKSs.

Check to see if your motherboard has an onboard RAID controller (for basic RAID0/1 functionality), an indicator of this would be that if you have an Intel Southbridge ICH8/9/10R or certain NVIDIA chipsets or even Gigabyte's own RAID controllers. Check your motherboard's specifications.

Well, it's not really "hardware" RAID, but rather, "Hardware-Assisted Software RAID" in that it still uses the processor to calculate striping (though it's not that much of a load compared to something like, say, RAID5 or 6). The RAID controller on your chip (if it even has one, snoop around your BIOS and see if you can find any enabling functions in the options) will only pertain to particular SATA ports, I haven't encountered any setup that can do mix-and-match controller-based RAID on a motherboard before, so I'm going to go slightly on a limb here and say "no" in that you can't mix-and-match IDE and SATA disks on your motherboard's RAID controllers.

Software-based would be good enough, though hardware setups would be preferable; your performance issues probably pertain more to how the partitions are split and the different disk sizes, areal densities, cache, and general performance bottlenecking the setup.