The graphics cards with the best price/performance are around the $150 mark, such as the GTX 460/550Ti/HD 6850.
Also, are you going to use Linux? AMD has open-source drivers, which are lax in features compared to nVidia, which uses a proprietary driver blob. Basically, with AMD on Linux you still have issues with desktop managers like Gnome 3 and horrible OpenGL performance (games like Minecraft use OpenGL). On the other hand, if you're looking to do parallel tasks like creating hashes and ray tracing, AMD may be a better option on Windows than nVidia because there is less distinction between its Radeon consumer line and its professional line, where nVidia really cripples the number of shaders in its GeForce line.
I would strongly recommend the HD 6850 for a discrete GPU at $150 +/-
1) Pretty power efficient (229W with a Lynnfield i7 on full load! ) compared to other graphics cards of comparable performance. You'll need a PSU with two PCIe power connectors though.
http://www.tomshardware.com/forum/312879-33-minimum-rail-required-radeon-hd6850 12-20 A on the 12V rail will suffice, it's that efficient.
http://www.bit-tech.net/hardware/graphics/2010/10/22/ati-radeon-hd-6850-review/92) Top price performance (has been top for more than 4 months already....)
http://www.videocardbenchmark.net/gpu.php?gpu=Radeon+HD+6850It will run everything on mid settings at 1080p with 2xAA, no question.
http://www.guru3d.com/article/radeon-hd-6850-6870-review/20$149.99 for the Sapphire version on Newegg.
http://www.overclockersclub.com/reviews/sapphire_hd6670_ultimate_hd6750_hd6770/6.htmI would recommend the HD 6770 for a card around $109, which outperforms the GTS 450 at the same price range. However, with this card, we are looking at just 60FPS at mid-high on most games at 1680x1050. 1080p gaming will be viable if you choose to turn all details down to low. It consumes 241 W on load with an i7 920/Asus P6T Deluxe, so most any power supply should be adequate. If you can find a HD 5770 for cheaper that would be great, as the 6770 and 5770 use the exact same graphics core (Juniper XT).
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150540 HD 6770 XFX ($119.99)
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125353 HD 6850 Gigabyte ($149.99)