the problem with the 3GB switch is that you reduce the memory that remains available for the device drivers. Usually 2GB can be used, when you set the 3GB switch the amount is reduced to 1GB. This space is shared by all drivers on the system. Depending on the devices in a system it might work well one one but fail on another system. I suppose when you see that DirectX fails due to this switch it is becaus the base driver runs out of memory. Actually there is not much you can do about it except trying some different values for userva or not using the 3GB switch. The amount of physical memory or available disk space has no influence since you only have 4GB of addressable memory.