The 590s are great. I had a pair for 2 years. I actually like them better than the 600s, which sound like they are trying to sound like a pair of Stax, but just don't do it. Your mac should be able to drive the 590s alright, I wouldn't worry about it -- you won't *need* an amp.
If you want cheap though, the Grado SR-60s really surprised me. You can probably find them new for about 60 bucks. The detail isn't quite as good as the 590s (well, not too close really at all), but they're still incredibly enjoyable to listen to, and can be driven very well by absolutely anything. I just got a pair for my girlfriend.