Anyone running Win98 is more likely to have a system that can't run Doom3 though...
Eh, not really. Besides myself, I know quite a few people that built their own PCs. Most will use (or, ahem, reuse) Windows 2000 because of no activation, while another large portion doesn't mind shelling out $100 USD for XP Home (or the $150 USD XP Pro OEM disc).
But I know a couple that will just install 98SE on their new hardware, mostly because they're cheap or just can't afford it.
98 and 2000 have no activation, so they are more appealing to the less fortunate (or enthusiastic, or anti-MS zealots).
So those few friends I have that rebuild their machines twice a year have pretty modern rigs (Athlon XP 2800, P4 3.x, etc), but their still running a P.O.S. operating system.
Pentium 4 3.06 GHz w/ Hyperthreading & 533 MHz FSB
1024MB PC2700 Crucial RAM
ATI Radeon 9800XT
Windows XP Pro
"Space. It seems to go on and on forever. But then you get to the end and a gorilla starts throwing barrels at you."