I'm gonna use a Socket 939 AMD64 3200+, with a GeForce 4 ti4200 in the AGP slot.
512 megs of kingston ram (ddr266 sadly)
This machine is going to have a dual boot between Windows XP and Windows 98se, and if I can figure it out, possibly a triple boot with Ubuntu as the third.
The GeForce 4 ti4200 is perfect for playing older games like..
Thief: The Dark Project
Thief 2: The Metal Age
System Shock 2
Final Fantasy 7 and 8
All of those games have known issues with newer hardware, in many cases making the game unplayable or just plain terrible looking.
98se will be nice for pretty much all dos games and older games that hate XP.
I'm gonna use a 160gb Maxtor hard drive. I'm thinking a 30gb FAT32 partition for 98se, and use the rest for XP (or split with ubuntu mind..)
So yeah, in general, it will be around for playing my older games and when I have friends over, it will do pretty nicely for playing CS: Source