An old refurbed machine would likely be plenty of computing power for a dedicated task like this. You can pick those up for almost nothing, I just traded half a dozen pentium 133 - amd 350's for a lunch each.
Windows 98 would be fine as well, you won't need the extra overhead of 2k/xp, and you wouldn't benifit much from them anyway.
No need to spend the extra dollars no wasted computer cycles. If the old system was slow, I highly doubt it was the computer speed, I would suspect it was a limitation of the machines, or the programming.