I'd grab either of the 9800GTs, the performance should be somewhat similar to your old card and it's a good step ahead of the 9600 which is only slightly cheaper.
If you're looking for performance, the HD4850 certainly has the best value out of the lot, however most of the benefit will only show in games like Crysis or with high resolutions - my 9600 gets me quite well through most games (Crysis being the exception of course) at 1440*900.
Overall if you don't mind the extra cash, I'd say the 4850 is the best investment considering the graphical fidelity of games will only increase, and there isn't much point saving money if you'll have to upgrade again in the future.
At any rate I'd strongly recommend against the 8600, they're already pretty outdated and will struggle with running anything remotely new on decent settings and resolution.
If you're looking for performance, the HD4850 certainly has the best value out of the lot, however most of the benefit will only show in games like Crysis or with high resolutions - my 9600 gets me quite well through most games (Crysis being the exception of course) at 1440*900.
Overall if you don't mind the extra cash, I'd say the 4850 is the best investment considering the graphical fidelity of games will only increase, and there isn't much point saving money if you'll have to upgrade again in the future.
At any rate I'd strongly recommend against the 8600, they're already pretty outdated and will struggle with running anything remotely new on decent settings and resolution.