In Picking a home PC platform I described my requirements for my ideal home PC platform. I've put together a "system formula" - a list of parts to use when building a machine.
The first decision to make is to select a CPU socket. This in turn confines you to a certain set of motherboards, a certain set of CPUs, and a certain type of memory. I ended up going with "Socket AM2".. It supports low-power "Manila" processors starting at 1.6GHz ($40), so the cost of entry is low. The fastest AMD processors are AM2, and even processors for the next socket (AM2+) are back-compatible with AM2 motherboards. It is likely to be viable for quite a while, giving my formula a long lifetime. AM2 sockets take DDR2 RAM.
The next question was motherboard form factor. FlexATX and mini-ITX are interesting, but there are many boards in those sizes. The only mini-ITX AM2 board I found was $300, too expensive for my purposes. There are lots of MicroATX (9.6" x 9.6") boards and cases out there, plus they fit in ATX cases, too.
With that in mind, I went motherboard shopping, and found the ASUS M2NPV-VM ($85). This board has a lot going for it:
Aside: why do they bother with PS2 and LPT ports on the board?
With so much built-in to the motherboard, there's not going to be a lot of deviation in the various roles.
There doesn't seem to be much point in configuring this machine with anything less than 1GB RAM, as buying a pair of 256M sticks doesn't save much money, and buying small RAM is always a waste. However, buying 667MHz RAM is probably find for a low-end machine. $66.
You can use any good quality case that suits your needs. $90 with PSU is normal.
Small hard disks aren't differentiated by price, so there's not much point in getting a tiny one, no matter what the role. A WD Caviar is a good choice for a basic box. 80GB = $45, although going up to 200GB is perfectly reasonable.
DVD writers are so cheap that there's not much point in skipping them, no matter what the role of the machine. (CD writers and DVD readers are no savings). You can even get them SATA today for only a couple bucks more. $35.
That's $360 for a basic machine. (If dollars were really tight, you could squeeze it down to $300, but I don't think it's a good idea.)
This machine would fill any of the roles I listed before, with only the following modifications:
SERVER - a slightly larger case + only 500GB or 750GB drives
GAMING - fast CPU, 800MHz RAM. Optionally a very fast CPU, a WD Raptor (10K RPM) system drive or two Caviars in RAID0, more RAM, and a very fast video card.
I don't know what the minimum requirements are for HD TV and DVD playback, but when the day comes that I care about that, I'm sure this platform will be easily upgradable to support it.