You really don't need that powerful of a graphics card to play the game and play it well, unless you insist on the absolute highest detail levels at a ridiculous screen resolution. If that's what you're going for, its going to cost you, but the game looks good enough that dropping down to 1024x768 with still high image quality settings is more than enough for most users. The high end cards of the last generation, such as the Radeon 9800 pro and Geforce 5900, should give you that much. They're not exactly cheap at just under $200, but that's far more acceptable than buying the latest and greatest.
As a side note, although I haven't played either one yet (new PC with standard Geforce 6800 is due to arrive any day now), I think the screenshots I've seen of Far Cry look better than Doom 3.