I have a latest gen 2013 iMac, and a high end PC with a Create X-FI Titanium Fatal1ty Pro Sound Card ($120). For my testing I used my Sennheiser HD 600's direct connected, no amp. I used Winamp on the PC and iTunes on the Mac, with the same sources, mostly MP3 at 320 kbps. I never use EQ, so this was off in all tests.
Both the Mac and X-FI have plenty to drive the 600's to dangerous levels, with no audible hum (SNR is equally good on both). I could not bear going beyond 70% volume on either one. But they did not sound the same to me at all. The Mac was noticeably more bass heavy, and lacked the sound stage and separation I heard on the X-FI. The mid's on the X-FI came through more, with a sparkle in the treble that the Mac was missing. In addition to this, although there was less bass quantity on the X-FI, I felt it was more composed and resulted in more definition. This became clear on songs with percussion instruments, you hear more detail with each stroke. Although I had to really listen analytically with each side by side to hear differences, they were clear.
I would like to say the iMac's DAC is substantially better than my PC's on-board. However, most audiophiles agree that in general, a DAC affects audible differences more so than an amp. I found this to be true with my experience connecting my Fiio E11 and Bravo V2 amps through both the iMac and PC. The differences I hear with amplification are much more subtle than the differences I hear between the iMac and X-FI DACs. Therefore, regardless whether you have a PC or a Mac, I still suggest upgrading your DAC before getting amplification.
I must disagree with bigshot completely that you cannot hear the difference between an iMac's DAC and a dedicated external one. I would venture to say bigshot probably doesn't own any high end DAC's himself. Just like with headphones, published specifications do not indicate very much about how a DAC will sound. There are several electrical design factors and materials that go into the execution of a quality DAC. There are numerous reviews of DAC's by Mac owners, like the Schiit Modi ($99). I have yet to read one that did not hear significant improvements. Note that some reviews have amps, but testing was done with the same amp on both the iMac and Modi. And this is just one example of a good sub $100 DAC.