what exacly does headphone amplifiers do?

okay okay...hold on to the money train a second-)

extras like headphone amps are never a must have. The more you spend on more gadgets the more your return for your money diminshes. Headphone amps are for high quality audiophile or entry level audiophile headphones who's impediance or resistance are too high for non-amplified jacks. And a bunch of other stuff....

Read this for a basic overview of what headhpone amps do and what headhpones can benefit from them check out THIS link and the rest of the website after you read this page.
<A HREF="http://headphone.com/layout.php?topicID=20&subTopicID=79" target="_new">http://headphone.com/layout.php?topicID=20&subTopicID=79</A>

For info on headphones for gaming read this link, also very important for maximum gaming pleasure.
<A HREF="http://headphone.com/layout.php?topicID=2&subTopicID=125" target="_new">http://headphone.com/layout.php?topicID=2&subTopicID=125</A>

I personally use the Sennheiser HD580 headphones and a Total Airhead 9v amp. For listening to music, i use that on a Sony Discman since it gives much better sound than either a TBSC, GTXP, or Audigy can possibly output for music(but then i'm the rare guy that doesnt pirate mp3's). Setup works great on either...but most people can't afford the 440 or so bucks to listen to use that kind of gear. Then, many people also don't have the ears to hear the difference. The difference is not night and day on a pc. It IS night and day on high quality audio equipment. Unless your buying very nice high quality headphones dont even THINK about an amp. Read everything on the headphone.com site you find interesting, its all very good and accurate information. But, keep in mind, when they talk about a big difference, its barely noticable on anything remotely related to gaming. Your dealing with extremely fine audio equipment and a pc cannot compare to the sound SOURCES those guys use. For a gamer just looking for the best sound for gaming i would recommend something like the Beyer headphones listed at the top of that second link i gave you. They're 169.00USD and work okay with or without an amp.

The itsy bitsy spider climbed up the empires state building, along came goblin, wiped the spider out

Agree with Ritesh but i'd like to add another category what he was sayin. PC Speaker sets, some of them have "amplified" headphone out jacks. The one on my Logitech Z-560's is mostly sufficient for any pc use. Wanted the absolute best sound i could get for listening to music that i could afford though for both at home and portable so i got the Total Airhead for myself because the HD 580's are built for use with an amp...normal people dont buy headphones like that...i'm wierd.

The itsy bitsy spider climbed up the empires state building, along came goblin, wiped the spider out

I cannot and will not attempt to answer that for you. I will say this, Headroom has a 100% money back 30 day satisfaction guarantee. (You end up paying postages though)
The one i use is actually 200+power adapter...not 100 bucks... Its very nice, but i use 580's and Senn 570's are a very different animal i havn't heard. Sennheiser headphones vary greatly between models except for the 580 and 600. Was a frivolous expense for me, dont recommend it unless your just really drooling to get exceptional quality music. I'm planning on getting a nicer dedicated cd player in a few months which is part of why i went ahead and got the amp.

The itsy bitsy spider climbed up the empires state building, along came goblin, wiped the spider out