if u can afford, i recommend PIX-515E, if u can get unrestricted license, it's even better bcos u can test failover (but u need to have two pix). but 506 would be sufficient for most of the features.
be aware that PIX os 7.0 is out (not sure if it supports all models, please check cisco website for more info). As cisco also upgraded CCSP exam on 19th Sep, it's most useful if you can find a pix that can support 7.0