that REALLY depends on what altcoin you want to mine. just about EVERY coin has at least one, if not multiple, pools to choose from. And with new altcoins being released quite literally on a daily basis, new pools are springing up every day as well. however, to simplify things a bit, you could check out coinex.pw. they are an exchange and mining pool all in one. they offer several options in the way of mining different coins. they have traditional pools that mine individual coins and they also offer two switchpools (sha-256 and scrypt) which automatically switch back and forth between coins to mine the coin which happens to be the most profitable at the moment. seeing as how you want to get into gpu mining, i would HIGHLY suggest choosing a scrypt based coin or the scrypt switchpool. sha-256 coins are just too fast and put too much wear and tear on your cards to mine efficiently with video cards. As for choosing a card to mine with, I would suggest purchasing an R9 280X. you can pick one up right now on amazon.com for around $300 (well within your price range). with one R9 280X, you will be able to pull somewhere in the range of 700-750 kh/s mining scrypt algorithm coins. and if you're willing to spend around $600 and buy two of them, you can be at over 1 mh/s. right now, those cards seem to be the best bang for the buck when it comes to gpus.

for a comparison of mining equipment, you can visit https://litecoin.info/Mining_hardware_comparison

there are currently several r9 280x's for sale on amazon.com. you can view a list of those here http://www.amazon.com/s/ref=nb_sb_ss_i_0_4?url=search-alias%3Daps&field-keywords=r9%20280x&sprefix=r9+2%2Caps%2C191
if you do decide to purchase one, i would suggest going with one made by MSI. they have extremely good cooling systems and are just overall well-built cards.
if you decide you want to build a dedicated mining rig, i suggest you also buy a good motherboard that's built for the task. the Asrock H81 is built specifically for mining purposes and is extremely cost effective at around $100 per unit. the H81 has space for 6 gpus to be installed. it has 1 main slot for a card to be mounted directly to the board and then 5 auxiliary slots for extra cards to be connected via PCI-e riser cables. you can view the listing for that motherboard here: http://www.amazon.com/ASRock-H81-BTC-Litecoin-motherboard/dp/B00HEUIS2G/ref=sr_1_5?ie=UTF8&qid=1394797815&sr=8-5&keywords=asrock

anyway, i hope all this info helps and doesn't overload you too much. good luck!