Very important question: What's the budget???
IMO a 5670 or 5770 is a good way to go as you get DX11 support which is great on a quad core CPU (DX11 brings multi threading to help accelerate GPUs, on top of tesslation and some other nice bits like lighting and advanced depth of field).
If you have $300 or $400 then a 5850 or 5870 are really great and efficient cards. For these high end cards the hierarchy is 5850 < GT470 < 5870 < GT480 < 5970.
A 4870 is a really good card, I think it's faster than a 5670 maybe than a 5770 too but only DX10 support.
9800GT is old tech, I'd look at (for Nvidia) the GT 250 or GT 260.
Definitely browse this article (Best GPU for the money from Toms Hardware): http://www.tomshardware.com/reviews/graphics-card-geforce-radeon,2646.html
IMO a 5670 or 5770 is a good way to go as you get DX11 support which is great on a quad core CPU (DX11 brings multi threading to help accelerate GPUs, on top of tesslation and some other nice bits like lighting and advanced depth of field).
If you have $300 or $400 then a 5850 or 5870 are really great and efficient cards. For these high end cards the hierarchy is 5850 < GT470 < 5870 < GT480 < 5970.
A 4870 is a really good card, I think it's faster than a 5670 maybe than a 5770 too but only DX10 support.
9800GT is old tech, I'd look at (for Nvidia) the GT 250 or GT 260.
Definitely browse this article (Best GPU for the money from Toms Hardware): http://www.tomshardware.com/reviews/graphics-card-geforce-radeon,2646.html