More importantly, what's your budget, then we can spec you up.
But hearing about your requirements:
Cpu:
Get a cheap dual core: Intel E2180 or something
Ram:
2gb ram for xp min, 3-4gb for vista ( 4 if you go 64 bit, 3 if you stay 32 bit) Doesn't really matter if it's ddr, ddr2 or ddr3, ddr2 is the cheapest but of course it depends on your board...
Gfx:
Ati rad 1950pro / Geforce 8600GT or something similar
rest:
400w good branded psu
250gb hdd ? ( small and cheap...)
Should be doable for around 500 € (if not much less now, mate bought an E6600/2gb/7600GS512mb pc for 450€ 6 months ago...
Will run any game to date on at least minimal settings, stuff like civ4 will fly with gfx maxed out...
Will last for minimal specs for (the newest) games for about 2-3 more years...