The integrated GF7100 will work adequately for most situations, even HD content at 720P... but not at 1080i/1080P.
As for the (affordable) nVidia based add-on cards...
GF 7600GS/GT - 1080i - ok, 1080P - no.
GF 8400 series - 1080i - no, 1080P - no.
GF 8500 series - 1080i - ok, 1080P - no
GF 8600 series - 1080i - good, 1080P - good.
As for the (affordable) ATI based add-on cards....
Radeon X1600 - 1080i - ok, 1080P - no.
Radeon X1600pro - 1080i - ok, 1080P - no.
Radeon X1900series - 1080i - ok, 1080P - ok.
Radeon HD2400series - 1080i - no, 1080P - no.
Radeon HD2600series - 1080i - good, 1080P - ok.
Radeon HD3400series - 1080i - no, 1080P - no. (The HD3400 series is an updated HD2400)
NOTE: These assessments are based on my personal testing... Your results may differ...
Beware of the the "marketing techniques" used to disguise the capabilities of the different cards... Just because the model has a higher number than the last model does not mean that it is a more capable card (ie: HD2400 vs HD3400).
visit
GPU Review... they provide specifications and comparisons of many different graphics cards and chipsets...