Hi Steven,
Unfortunately, this is one of those areas where the more you learn, the more options you'll come across, and the more confusing it gets.
If you haven't been there already, I'd suggest browsing through some of the threads at:
http://www.head-fi.org/forums/.
I considered buying a portable amp like this one
http://www.headphone.com/headphone-amps/amplifiers/headroom-total-bithead.php, but decided that the volume of my iPod Touch was sufficiently loud, so I can't give a review of that amp.
If volume is your biggest concern, you can also try to find a more efficient pair of heaphones or you can try boosting the volume of your MP3 files (assuming that's your format) by increasing the Replay Gain value with MC or by using the free program MP3 Gain. I use 92 dB as my baseline.
Also, if you have lossless files on your PC that you sent to your amp, don't expect your iPod files to sound as good if they are in a compressed format. You should be able to replace your Amplifier with a Receiver or another Amplifier for less than $600, but then it gets into a never ending debate over how much money it takes to get "optimal" sound. I hope others more knowledgeable than me will chime in with ideas, but the point is, depending on what you want to accomplish and how large your budget is, there are a multitude of solutions for enhancing your listening experience.