I would grab a 4890 easily over a 275, even a 285 if it is a 960 or 1000mhz version. Standard 4890's, which all OC past 900mhz can be found ~ 160 dollars.
4890's will certainly be more future proof than the GT200. For reference of how the last great ATI cards aged check this out:
X1950XTX, and to a lesser extent X1900xtx KILL the 7900 series in quite a few scenarios today. Shader heavy games are murder for the G71 series. I believe that future games will show similar results on 4890 vs GT200b. I run a GTX 285, and still would rather go 4890.
ATI's technology since r300, with the noticeable exception of the r600 series, has absolutely murdered nvidia's in longevity. At the time they perform the same, but down the road, nVidia always loses out. It seems that nVidia develops for todays games, and usually wins or ties ATI (winning more often, at least this generation, and won in x1800 vs 7800 & 7900), but down the road, ATI's vision is proven to be the superior one.