I have not yet found one of the low-end routers that permits this so for this special type of situation, I build one from an old workstation using LRP software (leaf.sourceforge.net). All you need is two NICs, a motherboard with minimal RAM (e.g. P100 with 32meg works fine), power supply/enclosure and floppy drive. It's a great way to recycle a workstation that otherwise is not worth much and it does give a huge amount of control over DHCP, NAT, firewall rules, etc.
I keep looking for a low-end router that gives me this flexibility but I haven't found it yet. If I don't need the flexibility, I use a D-Link or Linksys; otherwise I do it the LRP way.
hope this helps,
paul