I run my pfsense on N40L, I have added a dual nic and single nic so I have 4. I have 8GB of memory in mine - thinking of moving to 16.
Pfsense runs great on it as a vm under esxi - I would never go back to running my router directly on hardware.. With a vm, I can take as snapshot before I do anything with it and then can always roll back.. This is great when running development code, ie before 2.1 was released, etc..
Here are the cards I used
http://www.newegg.com/Product/Product.aspx?Item=N82E16833106036
dual
http://www.amazon.com/HP-412648-B21-PCI-Express-GigaBit-Adapter/dp/B000J3OPOU
Sold by amtech - I contacted them with my order number and they made sure I got the low profile bracket, etc. Great service from them.
So I run my NAS with 3 disks and my linux play vm and other vm on and off and have no problems with performance.. But there are always at min 3 vms on, etc. Thing rocks!!