For a single PCIe slot card, you have 2 choices right now for a good performing card: 9800GX2 for about $400, or wait for the 4870X2 which I expect to land at $550-$600 but that's pure speculation.
A single 4850 or 9800GTX will be 'OK' but you won't be maxing Crysis at 1680x1050..
29 FPS playable? Is that 29 FPS minimum, maximum, average? If average, what's the minimum? If minimum, what's the average? If maximum, hell no.