There shouldn't be any loss in speed for a Combo drive, now that mainstream Combo drives offer 48x or 52x CD burning.
Are you looking for an external drive for use with a laptop or something? Or do you need an internal drive for a PowerMac? If you only want an internal, I'd recommend the LG GCC-4480B, which is fully supported for everything in OS X. 48x CD burning, DVD reading, and supported for Finder and iTunes. They are about $40 on eBay for the bare drive, if you can find one.