first off, is there a reason why you did not continue this question in your original thread
http://www.tomshardware.com/answers/id-2580125/headphones-improved-amp.html ? your question is essentially the same one. please respond to your threads to post additional comments on your original question.
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depending on price (and country), you may want to look at the ad700x as well. going by amazon prices they are about $24 difference (with free shipping) though some of the cheaper places on the net have them at $90us. the 700x is slightly better and may be worth a $20 odd dollars increase but otherwise i'd go with the 500x.
do realize that the ad-500/700/900 are all bass light headphones. they have wide soundstages, very little bass response and a more analytic sound signature. this makes them in demand with pro gamers since they can tell exactly where sounds are coming from and the low bass means you can hear light footsteps easily. however, they are not what i would call the most "enjoyable" set of headphones due to the same reasons which make them one of the first choices for competitive play. they dont have much bass and the sound signature is not very "fun" although yes, taste can vary and some people do enjoy them in that manner.
now, about amplififers/dacs/soundcards: you should be able to power the ad500x from your onboard sound fine unless of course your onboard amp is rather weak (in which case it would have trouble with many headphones). here is a brief comparison of the choices (i will explain what is best for you after listing them)
*external dac+amp. more expensive, but has capability for less noise in the signal for quality sound since all processing is done out-of-case. this is best for people who dont care about virtual surround (since you lose the capability for it) and who have high end headphones or who want a portable solution.
*external amp used with internal/slot soundcard: lets you process sound inside your pc but exports it to an external amp. generally this setup is used when you want the features of an internal amp (generally either virtual surround or the user already has a high end soundcard) but needs an external amplifier to "boost" the signal due to having hard to drive headphones.
*internal soundcard: generally price efficient and many will power the 500x perfectly fine. the xonar dg at $27 (sometimes with mail in rebate for cheaper yet) has a very powerful amp, is better quality than most onboard sound and offers virtual surround capability (dolby headphone) which makes it a great card for those on a budget.
*onboard sound: quality of sound can vary quite a bit here. sometimes onboard sound works great, while other times the onboard is so bad you can hear static, buzzing, noise or distortion. generally i suggest trying headphones out on your onboard first to see if you hear such things before moving to a soundcard to save you money. if you need a soundcard or amp you can always buy one later once you figure out if you need one or not.
so, what is best for you? i suppose i should say, how will the ad500x benefit from the above? generally all you will gain from a more powerful amp or better dac is higher volume levels, less distortion and slightly more (but still very light) bass output. if you are on a desktop pc i would say try out your onboard first, then if you want more, you can try the xonar dg. if you are on a laptop then things get a bit more complicated (and pricey) if you wanted to keep virtual surround and not use the onboard to process at least.
the only way i'd suggest going external dac+amp is if you wanted just a bit more bass or sound quality (and have devices you can use it with) for on-the-go use and you completely wanted to give up on virtual surround. generally decent ones are not cheap though. the only way i'd go external amp (not dac+amp) is if you wanted to do the same as above but wanted to use onboard sound if it supported virtual or if quality was okay but it just did not output enough power. or, of course you could use a soundcard at home and a tiny cheap amp for on the go as well which works fine if you feel the need.
so...after hearing about the ad500x you find that you dont want something so light on bass and analytical? you might like the sennheiser hd558 which loses out on some of the soundstage and pinpoint accuracy but gains some bass. the 558 has similar gains from using a better soundcard or amp.
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in short: try them with your onboard sound first, if you dont like it try a soundcard like the xonar dg.
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