I also have a Sheeva plug; it has a drobo attached and is serving 2TB of nonsense over AFP, SMB, and NFS to my various devices. It also runs openvpn (UDP port 53!) bind9 for recursion, and a couple other things. It doesn't have much internal storage but it does have an SD card slot. I have a 2GB SD card mounted as /usr which helps a lot with all the packages I have installed. It comes with Ubuntu but you'll need to install another kernel or re-compile for NFS support unless the newer ones have already corrected this oversight.

Try Bitdefender. They have a free version but their paid version is cheap and includes licenses for 3 computers. Supports 64 bit vista and XP.