help with crappy slow NAT throughput in Debian Etch

[Lukesaber]

Hello!

I'm running an HP 500mhz with 192 mb ram. I've got Etch 4r4a running on it. My ISP is rogers and I'm on extreme (7mbit)

My previous network was using a WRT54G as the NAT with pretty good throughput, but I thought with a linux nat I could get more speed.

I have created an IPTables ruleset to filter, and installed two nic's in it. Other than the IPTables, I have a dhcp server running. So, basically no load.

Heres the problem:

My throughput (either on the nat box) or on any other computer in my house, is maxing out at 20 - 40kbps download, while I have full upload speed. I have tried running without an IPTables filter, I have tried replacing a network card, nothing helps. From reading other forums, I have installed a 10/100 switch in between the NAT and cable modem (which helped, a little).

I have tried forcing both network cards to full duplex 100T with no change. I have also re-ran all the network cable in my house, brand new. I've also tried increasing the default packet buffer limits as well.

The nic's in the computer are a 3com and a d-link (the realtek had errors)

My network is like this:
cable modem -> 10/100 switch -> linux boxx -> linksys wrt54g (as wap/switch) -> computers in house

So... I'm at a loss! how can I get back to 600k (or more) speeds with the perky advantage of running linux?

Any help is REALLY appreciated!
--
could it be the computer im trying to use as a nat?
its a bay-style case, (like the ones you put a monitor on) and the pci slots come off of a board that is slotted into the mainboard. could this style of bus seriously hamper throughput?
*scratches head*
i love the advantage of running a server instead of a plastic box...