That depends on what you will be doing with it and what is important to you and what you would be willing to spend. My choices was based on what was on sale locally at MicroCenter:
I chose the Gigabyte GA-EX58-UD3R on sale at $175
iCore7 920 was $200
6 GB OCZ Ram was $110
Quadro FX580 was $204
=========upgrade total was $689
I used my existing hdd(s) and case.... Killer system for $700!!
Ray