I think it depends a lot on the software you planned to used with it.
Mostly editting pics ? Then you will need some real -estate for bigger
display , a minimum of 17" with DVI, of course larger better but costly. You need to pay attention to the refresh rate quoted in msec , I think the standard now is like 10msec ( i could be wrong) but the lower the number the better.
RAM aka Memory - a min of 2GB
DVD-Writer - to backup or share all those precious pics !
Hard disk - SATA 160 or 250
Display - Nvidia if you plan to play games
But seriously either ATI or Nvidia you choose, get those with 256 or 512Bit - 256MB RAM
CPU / Motherboard - get the best , I would stick to Intel Core2Duo processor and the M/B which can support it.
Assuming you have limitless ca$h