Welcome aboard. The system you have should be more than enough to run as a core. The only question would be the TV card. Currently the only "Supported" cards are these:
Hauppagge PVR 150, 250, 500 for SD or capturing from a set top box (STB)
Or the Silicon Dust HD Homerun for HD, in the US, perhaps someone from the European front can comment on what is available over there. I know they have a few more DVB or HD options.
Also note that you will need two (2) network interfaces. One for the internet modem or router from your ISP, and one to the internal LinuxMCE network. I recommend a gigabit card for the internal network, and get a gigabit switch. Currently the system runs well on 100Mbit networks, but if you have to purchase new hardware for your build, might as well plan ahead.
A second thing to note, The only thing I would buy from Fiire is the Chief (remote control). Their systems are pretty much set up for their version of the project. There are many incompatibilities with their hardware, and our LinuxMCE software.
You can build fairly inexpensive MD's from scratch. A few have posted regarding the eeepc's and the Wind PC readily available from newegg.com or your favorite hardware re-seller. There are many posts in the forums regarding everything above.
Browse through the forums, look at the project wiki, and if you have questions that you can not find answers for there, most of us can be found daily at the irc channel, irc.freenode.net channel #linuxmce. Most of the answers I found were in the forums, but in a pinch, the irc channel is a great way to get answers/experiences.
Regards, and Welcome,