It doesn't matters if you have external Graphics Card or not. You can play GTA IV almost fine if your graphics card is non-dedicated like– Intel HD 3000 which is mostly inbuilt inside laptops & notebooks. If you have non-dedicated integrated Graphics card then you can force your RAM (Physical memory) to provide space to your Video memory maybe for your RAM atleast 500 MB. You will just need to use Commandline text “-availablevidmem”. If you already have 64 MB then you can double it by adding “2.0” followed by the above command.
Similarly, you can triple it by adding “3.0”.
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But according to what you said that you have 256 MB then you cannot play GTA IV well without this settings. It will lag, froze or crash. Because GTA IV, at the least Graphics settings still uses around 230 MB video memory which is near to your graphics limit will make GTA IV slower & worser.