Is your favorite game (or genre) CPU limited or GPU limited ?
Some games, need plenty of CPU. Others, need plenty of GPU.
First, you study the games (read reviews), to see what the
considered opinion of the reviewers is. You can use the
level of detail sliders in the game, to adjust for a
less-powerful computer.
If the game stutters a bit, you can overclock the CPU by
10%, and then retest. I was surprised how much difference
that made, as it stopped the stuttering in the game I was
playing. If I enabled anti-aliasing on the video card
(to smooth out the jaggies), even my current video card
doesn't handle that well at all, and responsiveness goes
out the window.
You can find benchmarks for video cards, to give
simple integer ratios between your old card and
a new one. As far as I know, 3DMark has a CPU component
to it, so part of the marks are for the CPU. (If the
card didn't give the same benchmark on your system,
it could be the CPU that's contributing to the loss.)
(I'd sooner trust these guys to run a bench, than Tomshardware.
At least with the tools they use, they have some control over
matching the test conditions. The tool should record the
hardware being used. I presume when they made the chart,
they selected similar other conditions for each entry.)
http://community.futuremark.com/hardware/gpu
Bench Ratio Approx_Price_New
HD 4770 1840
HD 6850 3580 1.95x $150
HD 6870 4200 2.28x
HD 6770 2550 1.39x
HD 7970 8830 4.80x $460
You can look up the individual card characteristics here.
The HD 6850 was from around the end of 2010, priced
around $150 or so.
http://www.gpureview.com/Radeon-HD-6850-card-636.html
High end video cards, one of their benefits, is being
able to either drive a panoramic LCD setup, or drive
a 2560x1600 30" monitor, that sort of thing. If your
monitor will always be at 1280x1024, then a mid-range
card may bring up the frame rate enough, to satisfy.
Paul