Depends on the RAID controller you use... Nvidia Nforce 4 & 500 seris built-in RAID will allow the use of IDE & SATA on the same array with most motherboard models, however I've never tried it personally & I'd think the IDE drive(s) would have the potential to slow the SATA drives down a bit. Also even if your RAID controller won't support combining the two types of drives directly, you can get IDE to SATA converters and just proceed as if you had all SATA drives to begin with.
The best bet is to consult the documentation that came with your motherboard or RAID controller for further information.