Nothing really, Altec has its own sound. Altec 14's run around $300-$500 a pair, about the same for the Altec 15's. In this price range you won't find any speaker that sounds as good. JBL's can be had for a little cheaper, but they won't sound "quite" the same, but similiar and yes much better than any offerings under $500.. I'd HIGHLY recommend the JBL L36's, they can be bought for $200-$300, there are some "poor man's" Altecs on audio.net, but they're called that for a reason, they are poor sounding speakers, but for $100 they might be worth a shot.
Here's a link,
http://members.aol.com/KDresch/sd2506.html But you need 5 of them RIGHT?.
If $500 is TOO much for you to spend on a pair of speakers, then you cannot really expect "score" something really great unless you happen to find a great deal somewhere. And you'll be spending well over a $1000 for all 5. A lot of people have been finding great deals on Craigslist, but I wouldn't count on it. But you could get lucky. But probably not 5 speakers.
I currently have for sale my old Large Adevent speakers which I used for a 5.1 set-up. They are all in nice conditon and have real walnut wood veneer, shipping would kill you though, but I'm not terribly far if you were wanting to pick them up. I have a subwoofer to go with them as well. Bought them for my parents to use for this very purpose. All the people that heard these speakers have raved about them. Me inculded, they are fantastic. The sound is different as compared to the Altecs. These are New England speakers, but that doesn't mean they sound bad. On the contrary, they are excellent sounding speakers for the money. They sell for $300-$400 a pair on Ebay in the condition they are in. One speaker needs a new crossover, but the rest are 100% functional with new foams. All the grills are in excellent condition.
I'd accept $700 for all 5. And would make you a GREAT deal on the subwoofer if you came and picked them all up... The subwoofer is a BRAND NEW HSU VTF-2. It cost $399 direct from the factory. I'd probably accept $900 for all 5 speakers and the subwoofer... There's no set-up anywhere at this price point with better sound.. I can assure you of that.. I KNOW you'll be very happy with them....
If you are able to build your own cabinets I'd highly recommend Audio Nirvana full range drivers, while they won't sound the same as the Altec's they will outperform them in almost everyway. The bass won't be quite as powerful or "boomy", but very satisfying and very realistic. I think you'll be pleased with them.. I for one would prefer them over the Altec 15's, and only the Altec 19's would sound better.
Below is a link the raw drivers, they start at $118 for a pair of the 8" Nirvanas. Don't bother with the 6.5"ers. The Super 8's are better, the 10's and 12's better still. I have heard these in person and I think you'll be pleasantly surprised if you are into the Altec as the sound is similiar. Don't let the fact that they are full range drivers fool you. Plenty of GREAT speaker systems only use 1 driver. He is a good friend of mine, and actually makes no money on the actual drivers, its the cabinets that he charges a little extra for. If your good at building cabinets than he's your man..
http://www.nirvanaspeakers.com/nirvana.html
Of course, I'd rather just sell you my speakers, but its not a big deal, its better for you know all the BEST options.. Let me know if you are interested in the Advents. They will be going on Ebay soon.. They are the original Large Advents not the cheaper 80's version.. The last pair I sold on Ebay went for over $500, so I'm not too worried if you're not interested in them, just thought I would offer them to you..
What receiver are you planning on using?