Of course GTX275 is beating 5850 in Dirt 2, because Dirt2 is running DX11 on 5850:
"The lack of DirectX 11 support gives some advantage to the Radeon HD 4890 and GeForce GTX 275, yet the Radeon HD 5830 is again much slower than the HD 5850 and just a little better than the HD 5770."
Far Cry 2 either runs better on NV hardware or doesn't benefit as much from 5xxx architectural improvements (depends on how you want to look at it):
http://ixbtlabs.com/articles3/i3dspeed/0210/itogi-video-fc-wxp-aaa-2560-pcie.html
^^^
5850 is only 3 frames faster than GTX275 (almost consistent with results I posted above). GTX285 is just 10 frames slower than 5870. So, of course a 480 SP GTX480 will give a 5870 a whooping of its life in a game where a 240 SP card is on the heels of a 5870...
Far Cry 2 @ Firingsquad:
http://www.firingsquad.com/hardware/ati_radeon_hd_5830_performance/page7.asp
^^^
Even GTX260 216 is beating a 5830!
5850 is only 12% faster than GTX 275 @ 1920x1200 4AA/8AF (55 vs. 49 frames). Think about it, 5850 is 20% faster than GTX285 and here it's just 12% faster than GTX275.
Why didn't they post 5870 vs. GTX480 @ STALKER: Call of Pripyat? Oh I know why, because that's a game that favours ATI 5xxx series:
http://www.firingsquad.com/hardware/ati_radeon_hd_5830_performance/page4.asp