I disagree. Taking from the prices at prophecy.co.za and the results from tomshardware with my rules:
Any game producing under 30fps for both is deemed "unplayable" at the resolution, and neither scores:
Total tests (via tomshardware): 67
Performed better by 9800GTX+ : 22
Performed better by 4870: 36
Unplayable: 9
Total Frames per second 9800GTX+ : 34172.8
Total Frames per second 4870 : 35794.1
This gives the 4870 a ~5% better than 9800GTX+
Power Consumption of the 9800GTX: 99w
Power consumption of the 4870: 117w
Price for the 9800GTX+ (was the evga, since then it's been removed, inno3d is cheaper though): R2467
Price for the 4870 (cheapest): R3242
This makes the 4870 ~31% more expensive.
Cost wise, 31% increase in price for a 5% increase shows that the 9800GTX+ is better value for money.
Having said that, the 4870 does outperform the 9800GTX+ at higher resolutions (1920x1200) but for lower resolutions the 9800GTX+ tends to do better. The fact that the GTX+ uses less power than the 4870 is a minor selling point to those of use who have to pay the power bills.
Cost is always an issue, if it wasn't we'd all be running tri-SLI 295's. Ultimately I opted for the 9800GTX+ (evga) because it was cheaper, and in the long run will save me a few cents on the power bill too and my screen cant handle those higher resolutions where the 4870 is better anyway.




