Can you recommend one for me for use with iPod (5th generation, video) and oh, I dunno, like Sennheiser PX100 or maybe a future pair of Grado SR80's that would make it sound really good?

Sorry this topic is really vague, I tried doing a search, but it wasn't real successful.

And are headphone amps really big and bulky? I'd prefer if it weren't. and do they make that much of a difference in terms of SQ.

You'd probably be better off to ask this question at www.head-fi.org in the amps forum - or simply search that forum, there are tons of amp recommendations for specific headphones there.
Good luck!

Two very cool options:

http://www.simpl.com
http://fixup.net

Have fun.

You can always make your own, look up 'mint headphone amps' and 'cmoy's

I myself am building a cmoy derivative, buffered, with a phone wall-wart as a power in (shielded by 2 large capacitors) and BUF-637s

For your portable arrangement, you might be pleased with a Ray Samuels Audio "Hornet".