The 506 is already marked end-of-sale/end-of-life so you'll need to get a new one anyway if you want support from Cisco. If you like the PIX (it's a good firewall anyway!) buy a bigger one but it doesn't hurt if you shop around abit. There are lots of other good alternatives.

Think about what you want or where you need it for. If you're going to buy a new one, think about the future too. You don't want to be stuck next year with an expensive firewall that doesn't have the features you need.