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Would I also need a dac if I am using my laptop's realtek soundcard?
That depends - on one hand, your laptop's soundcard was most likely not designed with the standards of an audio professional's or serious audio hobbyist's standards in mind; on the other, would you hear the difference? A lot of people have been disappointed that they can't, and come back here to vent their disappointment, but really, apart from listening to them yourself, there's really no other way to find out.
In any case, I'll elaborate on why it would be a good idea - a dedicated DAC is designed primarily to sound as much as possible like real music. While they all inevitably have a compromise, suffice to say the people designing the DAC, who are thinking of people who will compare and critically listen to their product, have less compromises to make with audio quality compared to the people who designed your laptop, who have many other performance parameters for a given MSRP and profit margin.
I haven't had enough time to critically listen to the HE500 myself though, so as to what specific gear would be great for it, I can't make any recommendations from personal experience. BUT, do keep in mind that it can demand more current, and whether you use just an amp, or also a separate DAC, or a device that can do both, make sure they can power the HE-500 first, even if you have to buy the amp first and worry about the DAC later. Although as someone who prefers skipping analog signal transfers, I'd say get the best DAC-Amp you can get.That said, you might wanna read this:
http://www.head-fi.org/t/527472/ibasso-d6-fer-de-lance-usb-dac-amp/495
