$500-$700 is a tight budget even with just the core components but you should be able to upgrade to a good mid range system:
gigabyte ds3 $134.99
C2D e6300 $184
g.skill ddr2800 1gb $114.99
WD sataII 250gb $74.99
FSP 450w $50.99
evga 7600gt $119.99
Total: $704.35 (w/ shipping)
Since you said you havn't upgraded in a long time, im guessing you have ddr memory. You will need ddr2 memory with the conroes, so you won't be able to use your old memory. The graphics card is a good mid-range card right now but it will quickly become a low end card after next year games. Evga has a step-up program
so you can trade in this graphics card for a mid range dx10 card that should be coming out early q1 next year.
Now you could get a 939 board with a dual core:
asus a8n-e $74.99
corsair xms ddr400 1gb $133
amd opteron 165 $153.99
Total (w/ the first parts minus the ram/cpu/mobo): $627.36 (w/ shipping)
The only reason i suggested the amd setup is because i went slightly over your budget the first time, and you can use you old ddr memory with it. But if you can handle the extra $4 with the first build and less memory, go with it.