First computer I bought was in the $1,000+ range.
The computer I'm on now I custom built this summer for about $500..
new case
new mobo (ABit KD7-Raid)
Athlon 2200+ XP
512 megs DDR Ram (400mhz)
GeForce FX 5600 Ultra (256 meg model)
I was able to save some money, because I had my existing 17" monitor and 120 gig hard drive (although I had to buy another 120 gig drive recently due to anime taking over my other one -_-)
It's fast enough for most of the stuff I do (casual gaming, web/c++ programming, video editing, running zillions of anime torrents), so I'm happy with it.
But to answer the question, it would depend on what I was buying the system for. If it was for gaming, I would go for the $1,500+ range.
If it was for more casual things, I would stay in the $1,000- range.