The athlon and the 2gb are perfect, it's what i have to play the game
For a mere 50€ you can pick up a 9800gt, which is what i use to run gta4.
You should run it in low. If you want medium, you can go for medium, but make sure your pc is cool..... my card reached 96ºc (around like 200ºf) (4 degrees below boiling temp in celcius).
The 6150 just wont cut it...
If you decide to pick up the 9800GT, make sure you have 2 power connectors (2 molex connectors, 4 pin) coming from your power supply that are available.