the 8600gt isnt a horrible bang for the buck if you get it on ebay for like $55, or on a rebate deal for $50 or so. those are both possible.
that said, the 3650/2600xt are about the same (2600xt is actually faster) but they compete with the 8600gt more or less. there really isnt an exact equivalent to the 8600gts... though a 3850 256mb would be better but more expensive by about $20.
best bang for the buck cards right now are generally the 8800gs and 9600gt depending on what res you want to run and what games you want to play. if you wanna run AA , and high res get the 9600gt over the 8800gs basically