here's a pretty decent review of
benchmarks in some games to give you an idea...
in 17 the games benched the summary of benchmarks...
raw performance (FPS)...
all resolutions...GTX285>GTX275>4890>GTX260(216)...
1024x768...GTX285>GTX275>4890>GTX260(216)...
1280x1024...GTX285>GTX275>4890>GTX260(216)...
1680x1050...GTX285>GTX275>4890>GTX260(216)...
1920x1200...GTX285>GTX275>4890>GTX260(216)...
performance per watt...
all resolutions...GTX285>GTX260(216)>4890>GTX275...
1024x768...GTX260(216)>GTX285>GTX275>4890...
1280x1024...GTX285>GTX260(216)>4890>GTX275...
1680x1050...GTX285>GTX260(216)>4890>GTX275...
1920x1200...GTX285>GTX260(216)>4890>GTX275...
performance per dollar...
all resolutions...GTX260(216)>GTX275>GTX285>4890...
1024x768...GTX260(216)>GTX275>GTX285>4890...
1280x1024...GTX260(216)>GTX275>GTX285>4890...
1680x1050...GTX260(216)>GTX275>GTX285>4890...
1920x1200...GTX260(216)>GTX275>GTX285>4890...
the benches and performance per watt sandwiched the 4890 between the GTX260(216) and the GTX275, but the cost per dollar drops it to the bottom of the heap, so I would compare it to the GTX275 more than either of the other two, the cost just doesn't make it worth it imo...