I have a very similar setup, mine has a 3.5" 1TB USB hard drive hooked to it. I think I'm using mysql. I'm using Transmission instead of rTorrent. Which webserver are you using? I had never used lighttpd before, but quite like it's small footprint now. I've got MediaTomb installed, to share the media I have on the 1TB drive with my uPnP devices (PS3 to be specific). And I always have a remote X session going when my desktop is booted, I run an app called "conky" that lets me monitor all of the system stats.
Oh, and I'm running Debian Lenny off my SD card, I think I'm only using 1.5gb of the 8gb card so far. My kernel is in NAND.. 188.8.131.52
I quite like my little plug.
I don't need X installed. I use apache (still looks quite light for me, and more familiar). For monitoring I manually look to logs, top, iotop, htop
I also have a swap part. of 300MB rarely used.
My gentoo has 3 GB (portage tree, kernel sources, development environment, ...) takes a bit more of space. And no X.